# Hash join

Hash join
You are encouraged to solve this task according to the task description, using any language you may know.

An inner join is an operation that combines two data tables into one table, based on matching column values. The simplest way of implementing this operation is the nested loop join algorithm, but a more scalable alternative is the hash join algorithm.

Implement the "hash join" algorithm, and demonstrate that it passes the test-case listed below.

You should represent the tables as data structures that feel natural in your programming language.

Guidance

The "hash join" algorithm consists of two steps:

1. Hash phase: Create a multimap from one of the two tables, mapping from each join column value to all the rows that contain it.
• The multimap must support hash-based lookup which scales better than a simple linear search, because that's the whole point of this algorithm.
• Ideally we should create the multimap for the smaller table, thus minimizing its creation time and memory size.
2. Join phase: Scan the other table, and find matching rows by looking in the multimap created before.

In pseudo-code, the algorithm could be expressed as follows:

let A = the first input table (or ideally, the larger one)
let B = the second input table (or ideally, the smaller one)
let jA = the join column ID of table A
let jB = the join column ID of table B
let MB = a multimap for mapping from single values to multiple rows of table B (starts out empty)
let C = the output table (starts out empty)

for each row b in table B:
place b in multimap MB under key b(jB)

for each row a in table A:
for each row b in multimap MB under key a(jA):
let c = the concatenation of row a and row b
place row c in table C

Test-case
Input Output
A =
Age Name
27 Jonah
18 Alan
28 Glory
18 Popeye
28 Alan
B =
Character Nemesis
Jonah Whales
Jonah Spiders
Alan Ghosts
Alan Zombies
Glory Buffy
jA = Name (i.e. column 1) jB = Character (i.e. column 0)
A.Age A.Name B.Character B.Nemesis
27 Jonah Jonah Whales
27 Jonah Jonah Spiders
18 Alan Alan Ghosts
18 Alan Alan Zombies
28 Glory Glory Buffy
28 Alan Alan Ghosts
28 Alan Alan Zombies

The order of the rows in the output table is not significant.
If you're using numerically indexed arrays to represent table rows (rather than referring to columns by name), you could represent the output rows in the form [[27, "Jonah"], ["Jonah", "Whales"]].

## AppleScript

Translation of: JavaScript

Native AppleScript records lack introspection, but from Yosemite onwards we can read and write them a little more flexibly through the Foundation classes. The vertical bars distinguish AppleScript reserved words (name and character here) from field name literal strings.

use framework "Foundation" -- Yosemite onwards, for record-handling functions -- HASH JOIN ----------------------------------------------------------------- -- hashJoin :: [Record] -> [Record] -> String -> [Record]on hashJoin(tblA, tblB, strJoin)    set {jA, jB} to splitOn("=", strJoin)     script instanceOfjB        on |λ|(a, x)            set strID to keyValue(x, jB)             set maybeInstances to keyValue(a, strID)            if maybeInstances is not missing value then                updatedRecord(a, strID, maybeInstances & {x})            else                updatedRecord(a, strID, [x])            end if        end |λ|    end script     set M to foldl(instanceOfjB, {name:"multiMap"}, tblB)     script joins        on |λ|(a, x)            set matches to keyValue(M, keyValue(x, jA))            if matches is not missing value then                script concat                    on |λ|(row)                        x & row                    end |λ|                end script                 a & map(concat, matches)            else                a            end if        end |λ|    end script     foldl(joins, {}, tblA)end hashJoin -- TEST ----------------------------------------------------------------------on run    set lstA to [¬        {age:27, |name|:"Jonah"}, ¬        {age:18, |name|:"Alan"}, ¬        {age:28, |name|:"Glory"}, ¬        {age:18, |name|:"Popeye"}, ¬        {age:28, |name|:"Alan"}]     set lstB to [¬        {|character|:"Jonah", nemesis:"Whales"}, ¬        {|character|:"Jonah", nemesis:"Spiders"}, ¬        {|character|:"Alan", nemesis:"Ghosts"}, ¬        {|character|:"Alan", nemesis:"Zombies"}, ¬        {|character|:"Glory", nemesis:"Buffy"}, ¬        {|character|:"Bob", nemesis:"foo"}]     hashJoin(lstA, lstB, "name=character")end run  -- RECORD FUNCTIONS ---------------------------------------------------------- -- keyValue :: String -> Record -> Maybe aon keyValue(rec, strKey)    set ca to current application    set v to (ca's NSDictionary's dictionaryWithDictionary:rec)'s ¬        objectForKey:strKey    if v is not missing value then        item 1 of ((ca's NSArray's arrayWithObject:v) as list)    else        missing value    end ifend keyValue -- updatedRecord :: Record -> String -> a -> Recordon updatedRecord(rec, strKey, varValue)    set ca to current application    set nsDct to (ca's NSMutableDictionary's dictionaryWithDictionary:rec)    nsDct's setValue:varValue forKey:strKey    item 1 of ((ca's NSArray's arrayWithObject:nsDct) as list)end updatedRecord  -- GENERIC FUNCTIONS --------------------------------------------------------- -- foldl :: (a -> b -> a) -> a -> [b] -> aon foldl(f, startValue, xs)    tell mReturn(f)        set v to startValue        set lng to length of xs        repeat with i from 1 to lng            set v to |λ|(v, item i of xs, i, xs)        end repeat        return v    end tellend foldl -- map :: (a -> b) -> [a] -> [b]on map(f, xs)    tell mReturn(f)        set lng to length of xs        set lst to {}        repeat with i from 1 to lng            set end of lst to |λ|(item i of xs, i, xs)        end repeat        return lst    end tellend map -- Lift 2nd class handler function into 1st class script wrapper -- mReturn :: Handler -> Scripton mReturn(f)    if class of f is script then        f    else        script            property |λ| : f        end script    end ifend mReturn -- splitOn :: Text -> Text -> [Text]on splitOn(strDelim, strMain)    set {dlm, my text item delimiters} to {my text item delimiters, strDelim}    set lstParts to text items of strMain    set my text item delimiters to dlm    return lstPartsend splitOn
Output:
{{age:27, |name|:"Jonah", |character|:"Jonah", nemesis:"Whales"},
{age:27, |name|:"Jonah", |character|:"Jonah", nemesis:"Spiders"},
{age:18, |name|:"Alan", |character|:"Alan", nemesis:"Ghosts"},
{age:18, |name|:"Alan", |character|:"Alan", nemesis:"Zombies"},
{age:28, |name|:"Glory", |character|:"Glory", nemesis:"Buffy"},
{age:28, |name|:"Alan", |character|:"Alan", nemesis:"Ghosts"},
{age:28, |name|:"Alan", |character|:"Alan", nemesis:"Zombies"}}

 # syntax: GAWK -f HASH_JOIN.AWK [-v debug={0|1}] TABLE_A TABLE_B## sorting:#   PROCINFO["sorted_in"] is used by GAWK#   SORTTYPE is used by Thompson Automation's TAWK#BEGIN {    FS = ","    PROCINFO["sorted_in"] = "@ind_str_asc" ; SORTTYPE = 1    if (ARGC-1 != 2) {      print("error: incorrect number of arguments") ; errors++      exit # go to END    }}{   if (NR == FNR) { # table A      if (FNR == 1) {        a_head = prefix_column_names("A")        next      }      a_arr[$2][$1] = $0 # [name][age] } if (NR != FNR) { # table B if (FNR == 1) { b_head = prefix_column_names("B") next } b_arr[$1][$2] =$0 # [character][nemesis]    }}END {    if (errors > 0) { exit(1) }    if (debug == 1) {      dump_table(a_arr,a_head)      dump_table(b_arr,b_head)    }    printf("%s%s%s\n",a_head,FS,b_head) # table heading    for (i in a_arr) {      if (i in b_arr) {        for (j in a_arr[i]) {          for (k in b_arr[i]) {            print(a_arr[i][j] FS b_arr[i][k]) # join table A & table B          }        }      }    }    exit(0)}function dump_table(arr,heading,  i,j) {    printf("%s\n",heading)    for (i in arr) {      for (j in arr[i]) {        printf("%s\n",arr[i][j])      }    }    print("")}function prefix_column_names(p,  tmp) {    tmp = p "." $0 gsub(/,/,"&" p ".",tmp) return(tmp)}  TABLE_A input: Age,Name 27,Jonah 18,Alan 28,Glory 18,Popeye 28,Alan  TABLE_B input: Character,Nemesis Jonah,Whales Jonah,Spiders Alan,Ghosts Alan,Zombies Glory,Buffy  Output: A.Age,A.Name,B.Character,B.Nemesis 18,Alan,Alan,Ghosts 18,Alan,Alan,Zombies 28,Alan,Alan,Ghosts 28,Alan,Alan,Zombies 28,Glory,Glory,Buffy 27,Jonah,Jonah,Spiders 27,Jonah,Jonah,Whales  ## Bracmat This solution creates a hash table for the smaller relation in the function join. This function takes as arguments the smallest table, the biggest table and then three pieces of code: two patterns that describe each table's field order and code that generates one row of output. These pieces of code are inserted in a fixed skeleton of code using macro substitution. ( (27.Jonah) (18.Alan) (28.Glory) (18.Popeye) (28.Alan) : ?table-A& (Jonah.Whales) (Jonah.Spiders) (Alan.Ghosts) (Alan.Zombies) (Glory.Buffy) : ?table-B& new$hash:?H& !table-A:? [?lenA& !table-B:? [?lenB& ( join  =     smalltab bigtab smallschema bigschema joinschema      , key val val2 keyval2    .     !arg        : (?smalltab.?bigtab.(=?smallschema.?bigschema.?joinschema))      & :?rel      & !(         ' (   whl             ' ( !smalltab:$smallschema ?smalltab & (H..insert)$(!key.!val)               )           &   whl             ' ( !bigtab:$bigschema ?bigtab & ( (H..find)$!key:?keyval2                   &   whl                     ' ( !keyval2:(?key.?val2) ?keyval2                       & $joinschema !rel:?rel ) | ) ) ) ) & !rel )& out$ ( join    $( !lenA:~<!lenB & ( !table-B . !table-A . ( = (?key.?val).(?val.?key).!val.!key.!val2 ) ) | ( !table-A . !table-B . (=(?val.?key).(?key.?val).!val2.!key.!val) ) ) )&); Output:  (28.Alan.Ghosts) (28.Alan.Zombies) (28.Glory.Buffy) (18.Alan.Ghosts) (18.Alan.Zombies) (27.Jonah.Whales) (27.Jonah.Spiders) ## C++ #include <iostream>#include <string>#include <vector>#include <unordered_map> using tab_t = std::vector<std::vector<std::string>>;tab_t tab1 {// Age Name {"27", "Jonah"}, {"18", "Alan"}, {"28", "Glory"}, {"18", "Popeye"}, {"28", "Alan"}}; tab_t tab2 {// Character Nemesis {"Jonah", "Whales"}, {"Jonah", "Spiders"}, {"Alan", "Ghosts"}, {"Alan", "Zombies"}, {"Glory", "Buffy"}}; std::ostream& operator<<(std::ostream& o, const tab_t& t) { for(size_t i = 0; i < t.size(); ++i) { o << i << ":"; for(const auto& e : t[i]) o << '\t' << e; o << std::endl; } return o;} tab_t Join(const tab_t& a, size_t columna, const tab_t& b, size_t columnb) { std::unordered_multimap<std::string, size_t> hashmap; // hash for(size_t i = 0; i < a.size(); ++i) { hashmap.insert(std::make_pair(a[i][columna], i)); } // map tab_t result; for(size_t i = 0; i < b.size(); ++i) { auto range = hashmap.equal_range(b[i][columnb]); for(auto it = range.first; it != range.second; ++it) { tab_t::value_type row; row.insert(row.end() , a[it->second].begin() , a[it->second].end()); row.insert(row.end() , b[i].begin() , b[i].end()); result.push_back(std::move(row)); } } return result;} int main(int argc, char const *argv[]){ using namespace std; int ret = 0; cout << "Table A: " << endl << tab1 << endl; cout << "Table B: " << endl << tab2 << endl; auto tab3 = Join(tab1, 1, tab2, 0); cout << "Joined tables: " << endl << tab3 << endl; return ret;}  Output: Table A: 0: 27 Jonah 1: 18 Alan 2: 28 Glory 3: 18 Popeye 4: 28 Alan Table B: 0: Jonah Whales 1: Jonah Spiders 2: Alan Ghosts 3: Alan Zombies 4: Glory Buffy Joined tables: 0: 27 Jonah Jonah Whales 1: 27 Jonah Jonah Spiders 2: 28 Alan Alan Ghosts 3: 18 Alan Alan Ghosts 4: 28 Alan Alan Zombies 5: 18 Alan Alan Zombies 6: 28 Glory Glory Buffy  ## C# using LINQ to Objects using System;using System.Collections.Generic;using System.Linq; namespace HashJoin{ public class AgeName { public AgeName(byte age, string name) { Age = age; Name = name; } public byte Age { get; private set; } public string Name { get; private set; } } public class NameNemesis { public NameNemesis(string name, string nemesis) { Name = name; Nemesis = nemesis; } public string Name { get; private set; } public string Nemesis { get; private set; } } public class DataContext { public DataContext() { AgeName = new List<AgeName>(); NameNemesis = new List<NameNemesis>(); } public List<AgeName> AgeName { get; set; } public List<NameNemesis> NameNemesis { get; set; } } public class AgeNameNemesis { public AgeNameNemesis(byte age, string name, string nemesis) { Age = age; Name = name; Nemesis = nemesis; } public byte Age { get; private set; } public string Name { get; private set; } public string Nemesis { get; private set; } } class Program { public static void Main() { var data = GetData(); var result = ExecuteHashJoin(data); WriteResultToConsole(result); } private static void WriteResultToConsole(List<AgeNameNemesis> result) { result.ForEach(ageNameNemesis => Console.WriteLine("Age: {0}, Name: {1}, Nemesis: {2}", ageNameNemesis.Age, ageNameNemesis.Name, ageNameNemesis.Nemesis)); } private static List<AgeNameNemesis> ExecuteHashJoin(DataContext data) { return (data.AgeName.Join(data.NameNemesis, ageName => ageName.Name, nameNemesis => nameNemesis.Name, (ageName, nameNemesis) => new AgeNameNemesis(ageName.Age, ageName.Name, nameNemesis.Nemesis))) .ToList(); } private static DataContext GetData() { var context = new DataContext(); context.AgeName.AddRange(new [] { new AgeName(27, "Jonah"), new AgeName(18, "Alan"), new AgeName(28, "Glory"), new AgeName(18, "Popeye"), new AgeName(28, "Alan") }); context.NameNemesis.AddRange(new[] { new NameNemesis("Jonah", "Whales"), new NameNemesis("Jonah", "Spiders"), new NameNemesis("Alan", "Ghosts"), new NameNemesis("Alan", "Zombies"), new NameNemesis("Glory", "Buffy") }); return context; } }} Output: Age: 27, Name: Jonah, Nemesis: Whales Age: 27, Name: Jonah, Nemesis: Spiders Age: 18, Name: Alan, Nemesis: Ghosts Age: 18, Name: Alan, Nemesis: Zombies Age: 28, Name: Glory, Nemesis: Buffy Age: 28, Name: Alan, Nemesis: Ghosts Age: 28, Name: Alan, Nemesis: Zombies  ## Clojure  (defn hash-join [table1 col1 table2 col2] (let [hashed (group-by col1 table1)] (flatten (for [r table2] (for [s (hashed (col2 r))] (merge s r)))))) (def s '({:age 27 :name "Jonah"} {:age 18 :name "Alan"} {:age 28 :name "Glory"} {:age 18 :name "Popeye"} {:age 28 :name "Alan"})) (def r '({:nemesis "Whales" :name "Jonah"} {:nemesis "Spiders" :name "Jonah"} {:nemesis "Ghosts" :name "Alan"} {:nemesis "Zombies" :name "Alan"} {:nemesis "Buffy" :name "Glory"})) (pprint (sort-by :name (hash-join s :name r :name)))  Output: ({:nemesis "Ghosts", :age 18, :name "Alan"} {:nemesis "Ghosts", :age 28, :name "Alan"} {:nemesis "Zombies", :age 18, :name "Alan"} {:nemesis "Zombies", :age 28, :name "Alan"} {:nemesis "Buffy", :age 28, :name "Glory"} {:nemesis "Whales", :age 27, :name "Jonah"} {:nemesis "Spiders", :age 27, :name "Jonah"})  ## Common Lisp (defparameter *table-A* '((27 "Jonah") (18 "Alan") (28 "Glory") (18 "Popeye") (28 "Alan"))) (defparameter *table-B* '(("Jonah" "Whales") ("Jonah" "Spiders") ("Alan" "Ghosts") ("Alan" "Zombies") ("Glory" "Buffy"))) ;; Hash phase(defparameter *hash-table* (make-hash-table :test #'equal)) (loop for (i r) in *table-A* for value = (gethash r *hash-table* (list nil)) do (setf (gethash r *hash-table*) value) (push (list i r) (first value))) ;; Join phase (loop for (i r) in *table-B* do (let ((val (car (gethash i *hash-table*)))) (loop for (a b) in val do (format t "{~a ~a} {~a ~a}~%" a b i r)))) Output: {27 Jonah} {Jonah Whales} {27 Jonah} {Jonah Spiders} {28 Alan} {Alan Ghosts} {18 Alan} {Alan Ghosts} {28 Alan} {Alan Zombies} {18 Alan} {Alan Zombies} {28 Glory} {Glory Buffy}  ## D Translation of: Python import std.stdio, std.typecons; auto hashJoin(size_t index1, size_t index2, T1, T2) (in T1[] table1, in T2[] table2) pure /*nothrow*/ @safeif (is(typeof(T1.init[index1]) == typeof(T2.init[index2]))) { // Hash phase. T1[][typeof(T1.init[index1])] h; foreach (const s; table1) h[s[index1]] ~= s; // Join phase. Tuple!(const T1, const T2)[] result; foreach (const r; table2) foreach (const s; h.get(r[index2], null)) // Not nothrow. result ~= tuple(s, r); return result;} void main() { alias T = tuple; immutable table1 = [T(27, "Jonah"), T(18, "Alan"), T(28, "Glory"), T(18, "Popeye"), T(28, "Alan")]; immutable table2 = [T("Jonah", "Whales"), T("Jonah", "Spiders"), T("Alan", "Ghosts"), T("Alan", "Zombies"), T("Glory", "Buffy")]; foreach (const row; hashJoin!(1, 0)(table1, table2)) writefln("(%s, %5s) (%5s, %7s)", row[0][], row[1][]);} Output: (27, Jonah) (Jonah, Whales) (27, Jonah) (Jonah, Spiders) (18, Alan) ( Alan, Ghosts) (28, Alan) ( Alan, Ghosts) (18, Alan) ( Alan, Zombies) (28, Alan) ( Alan, Zombies) (28, Glory) (Glory, Buffy) ## Déjà Vu Translation of: Python hashJoin table1 index1 table2 index2: local :h {} # hash phase for s in table1: local :key s! index1 if not has h key: set-to h key [] push-to h! key s # join phase [] for r in table2: for s in copy h! r! index2: push-through swap [ s r ] local :table1 [ [ 27 "Jonah" ] [ 18 "Alan" ] [ 28 "Glory" ] [ 18 "Popeye" ] [ 28 "Alan" ] ]local :table2 [ [ "Jonah" "Whales" ] [ "Jonah" "Spiders" ] [ "Alan" "Ghosts" ] [ "Alan" "Zombies" ] [ "Glory" "Buffy" ] ] for row in hashJoin table1 1 table2 0: !. row Output: [ [ 27 "Jonah" ] [ "Jonah" "Whales" ] ] [ [ 27 "Jonah" ] [ "Jonah" "Spiders" ] ] [ [ 28 "Alan" ] [ "Alan" "Ghosts" ] ] [ [ 18 "Alan" ] [ "Alan" "Ghosts" ] ] [ [ 28 "Alan" ] [ "Alan" "Zombies" ] ] [ [ 18 "Alan" ] [ "Alan" "Zombies" ] ] [ [ 28 "Glory" ] [ "Glory" "Buffy" ] ]  ## EchoLisp Since this is a real, professional application, we build the hash tables in permanent (local) storage.  (define ages '((27 "Jonah") (18 "Alan") (28 "Glory") (18 "Popeye") (28 "Alan")))(define nemesis '(("Jonah" "Whales") ("Jonah" "Spiders") ("Alan" "Ghosts") ("Alan" "Zombies") ("Glory" "Buffy"))) ;; table: table name;; source : input list;; key-proc : procedure returning the join value ('name' in this task) (define (table-hash table source key-proc )(local-make-store table)(for ((r source)) (local-put-value (key-proc r) (append (list r) (local-get-value (key-proc r) table)) table))) ;; build the two tables(define-syntax-rule (second record) (cadr record))(define (key-name-age record) (second record))(table-hash 'AGES ages key-name-age) (define (key-nemesis-name record) (first record))(table-hash 'NEMESIS nemesis key-nemesis-name) ;; join(for* ((k (local-keys 'AGES)) (a (local-get-value k 'AGES)) (n (local-get-value k 'NEMESIS))) (writeln a n))  Output:  (28 "Alan") ("Alan" "Zombies") (28 "Alan") ("Alan" "Ghosts") (18 "Alan") ("Alan" "Zombies") (18 "Alan") ("Alan" "Ghosts") (28 "Glory") ("Glory" "Buffy") (27 "Jonah") ("Jonah" "Spiders") (27 "Jonah") ("Jonah" "Whales")  ## ECL  LeftRec := RECORD UNSIGNED1 Age; STRING6 Name;END; LeftFile := DATASET([{27,'Jonah'},{18,'Alan'},{28,'Glory'},{18,'Popeye'},{28,'Alan'}],LeftRec); RightRec := RECORD STRING6 Name; STRING7 Nemesis;END; RightFile := DATASET([{'Jonah','Whales'},{'Jonah','Spiders'},{'Alan','Ghosts'},{'Alan','Zombies'},{'Glory','Buffy'}], RightRec); HashJoin := JOIN(LeftFile,RightFile,Left.Name = RIGHT.Name,HASH); HashJoin; //The HASH JOIN is built-in to the ECL JOIN by using the HASH JOIN Flag /*OUTPUT:Age Name Nemesis18 Alan Ghosts 18 Alan Zombies28 Alan Ghosts 28 Alan Zombies28 Glory Buffy 27 Jonah Whales 27 Jonah Spiders*/  ## Elixir Translation of: Ruby defmodule Hash do def join(table1, index1, table2, index2) do h = Enum.group_by(table1, fn s -> elem(s, index1) end) Enum.flat_map(table2, fn r -> Enum.map(h[elem(r, index2)], fn s -> {s, r} end) end) endend table1 = [{27, "Jonah"}, {18, "Alan"}, {28, "Glory"}, {18, "Popeye"}, {28, "Alan"}]table2 = [{"Jonah", "Whales"}, {"Jonah", "Spiders"}, {"Alan", "Ghosts"}, {"Alan", "Zombies"}, {"Glory", "Buffy"}]Hash.join(table1, 1, table2, 0) |> Enum.each(&IO.inspect &1) Output: {{27, "Jonah"}, {"Jonah", "Whales"}} {{27, "Jonah"}, {"Jonah", "Spiders"}} {{28, "Alan"}, {"Alan", "Ghosts"}} {{18, "Alan"}, {"Alan", "Ghosts"}} {{28, "Alan"}, {"Alan", "Zombies"}} {{18, "Alan"}, {"Alan", "Zombies"}} {{28, "Glory"}, {"Glory", "Buffy"}}  ## Erlang  -module( hash_join ). -export( [task/0] ). task() -> Table_1 = [{27, "Jonah"}, {18, "Alan"}, {28, "Glory"}, {18, "Popeye"}, {28, "Alan"}], Table_2 = [{"Jonah", "Whales"}, {"Jonah", "Spiders"}, {"Alan", "Ghosts"}, {"Alan", "Zombies"}, {"Glory", "Buffy"}], Dict = lists:foldl( fun dict_append/2, dict:new(), Table_1 ), lists:flatten( [dict_find( X, Dict ) || X <- Table_2] ). dict_append( {Key, Value}, Acc ) -> dict:append( Value, {Key, Value}, Acc ). dict_find( {Key, Value}, Dict ) -> dict_find( dict:find(Key, Dict), Key, Value ). dict_find( error, _Key, _Value ) -> [];dict_find( {ok, Values}, Key, Value ) -> [{X, {Key, Value}} || X <- Values].  Output: 15> hash_join:task(). [{{27,"Jonah"},{"Jonah","Whales"}}, {{27,"Jonah"},{"Jonah","Spiders"}}, {{18,"Alan"},{"Alan","Ghosts"}}, {{28,"Alan"},{"Alan","Ghosts"}}, {{18,"Alan"},{"Alan","Zombies"}}, {{28,"Alan"},{"Alan","Zombies"}}, {{28,"Glory"},{"Glory","Buffy"}}]  ## F# [<EntryPoint>]let main argv = let table1 = [27, "Jonah"; 18, "Alan"; 28, "Glory"; 18, "Popeye"; 28, "Alan"] let table2 = ["Jonah", "Whales"; "Jonah", "Spiders"; "Alan", "Ghosts"; "Alan", "Zombies"; "Glory", "Buffy"] let hash = Seq.groupBy (fun r -> snd r) table1 table2 |> Seq.collect (fun r -> hash |> Seq.collect (fun kv -> if (fst r) <> (fst kv) then [] else (Seq.map (fun x -> (x, r)) (snd kv)) |> Seq.toList) ) |> Seq.toList |> printfn "%A" 0 Output: [((27, "Jonah"), ("Jonah", "Whales")); ((27, "Jonah"), ("Jonah", "Spiders")); ((18, "Alan"), ("Alan", "Ghosts")); ((28, "Alan"), ("Alan", "Ghosts")); ((18, "Alan"), ("Alan", "Zombies")); ((28, "Alan"), ("Alan", "Zombies")); ((28, "Glory"), ("Glory", "Buffy"))] ## Forth Works with: Forth Works with any ANS Forth Needs the FMS-SI (single inheritance) library code located here: http://soton.mpeforth.com/flag/fms/index.html  include FMS-SI.finclude FMS-SILib.f \ Since the same join attribute, Name, occurs more than once\ in both tables for this problem we need a hash table that\ will accept and retrieve multiple identical keys if we want\ an efficient solution for large tables. We make use\ of the hash collision handling feature of class hash-table. \ Subclass hash-table-m allows multiple entries with the same key.\ After a get: hit one can inspect for additional entries with\ the same key by using next: until false is returned. :class hash-table-m <super hash-table \ called within insert: method in superclass :m (do-search): ( node hash -- idx hash false ) swap drop idx @ swap false ;m :m next: ( -- val true | false ) last-node @ dup if begin ( node ) next: dup while dup [email protected]: @: key-addr @ key-len @ compare 0= if dup last-node ! [email protected]: true exit then repeat then ;m;class \ begin hash phase : obj ( addr len -- obj ) heap> string+ dup >r !: r> ; hash-table-m R 1 r init s" Whales " obj s" Jonah" r insert:s" Spiders " obj s" Jonah" r insert:s" Ghosts " obj s" Alan" r insert:s" Buffy " obj s" Glory" r insert: s" Zombies " obj s" Alan" r insert:s" Vampires " obj s" Jonah" r insert:\ end hash phase \ create Age Name table So{ o{ 27 'Jonah' } o{ 18 'Alan' } o{ 28 'Glory' } o{ 18 'Popeye' } o{ 28 'Alan' } } value s \ Q is a place to store the relationobject-list2 Q \ join phase: join \ { obj | list -- } 0 locals| list obj | 1 obj at: @: r get: \ hash the join-attribute and search table r if \ we have a match, so concatenate and save in q heap> object-list2 to list list q add: \ start a new sub-list in q 0 obj at: copy: list add: \ place age from list s in q 1 obj at: copy: list add: \ place join-attribute (name) from list s in q ( str-obj ) copy: list add: \ place first nemesis in q begin r next: \ check for more nemeses while ( str-obj ) copy: list add: \ place next nemesis in q repeat then ; : probe begin s each: \ for each tuple object in s while ( obj ) join \ pass the object to function join repeat ; probe \ execute the probe function q p: \ print the saved relation \ free allocated memorys <free r free2:q free:  Output: o{ o{ 27 'Jonah' Whales Spiders Vampires } o{ 18 'Alan' Ghosts Zombies } o{ 28 'Glory' Buffy } o{ 28 'Alan' Ghosts Zombies } }  ## Go package main import "fmt" func main() { tableA := []struct { value int key string }{ {27, "Jonah"}, {18, "Alan"}, {28, "Glory"}, {18, "Popeye"}, {28, "Alan"}, } tableB := []struct { key string value string }{ {"Jonah", "Whales"}, {"Jonah", "Spiders"}, {"Alan", "Ghosts"}, {"Alan", "Zombies"}, {"Glory", "Buffy"}, } // hash phase h := map[string][]int{} for i, r := range tableA { h[r.key] = append(h[r.key], i) } // join phase for _, x := range tableB { for _, a := range h[x.key] { fmt.Println(tableA[a], x) } }} Output: {27 Jonah} {Jonah Whales} {27 Jonah} {Jonah Spiders} {18 Alan} {Alan Ghosts} {28 Alan} {Alan Ghosts} {18 Alan} {Alan Zombies} {28 Alan} {Alan Zombies} {28 Glory} {Glory Buffy}  ## Groovy Semi-imperative style:  def hashJoin(table1, col1, table2, col2) { def hashed = table1.groupBy { s -> s[col1] } def q = [] as Set table2.each { r -> def join = hashed[r[col2]] join.each { s -> q << s.plus(r) } } q}  More functional style:  def hashJoin(table1, col1, table2, col2) { def hashed = table1.groupBy { s -> s[col1] } table2.collect { r -> hashed[r[col2]].collect { s -> s.plus(r) } }.flatten()}  Sample run (either version as the result is the same):  def s = [[age: 27, name: 'Jonah'], [age: 18, name: 'Alan'], [age: 28, name: 'Glory'], [age: 18, name: 'Popeye'], [age: 28, name: 'Alan']] def r = [[name: 'Jonah', nemesis: 'Whales'], [name: 'Jonah', nemesis: 'Spiders'], [name: 'Alan', nemesis: 'Ghosts'], [name: 'Alan', nemesis: 'Zombies'], [name: 'Glory', nemesis: 'Buffy']] hashJoin(s, "name", r, "name").sort {it.name}.each { println it }  produces: [age:18, name:Alan, nemesis:Ghosts] [age:28, name:Alan, nemesis:Ghosts] [age:18, name:Alan, nemesis:Zombies] [age:28, name:Alan, nemesis:Zombies] [age:28, name:Glory, nemesis:Buffy] [age:27, name:Jonah, nemesis:Whales] [age:27, name:Jonah, nemesis:Spiders]  ## Haskell The ST monad allows us to utilise mutable memory behind a referentially transparent interface, allowing us to use hashtables (efficiently). Our hashJoin function takes two lists and two selector functions. Placing all relations with the same selector value in a list in the hashtable allows us to join many to one/many relations. {-# LANGUAGE LambdaCase, TupleSections #-}import qualified Data.HashTable.ST.Basic as Himport Data.Hashableimport Control.Monad.STimport Control.Monadimport Data.STRef hashJoin :: (Eq k, Hashable k) => [t] -> (t -> k) -> [a] -> (a -> k) -> [(t, a)]hashJoin xs fx ys fy = runST$ do  l <- newSTRef []  ht <- H.new  forM_ ys $\y -> H.insert ht (fy y) =<< (H.lookup ht (fy y) >>= \case Nothing -> return [y] Just v -> return (y:v)) forM_ xs$ \x -> do    H.lookup ht (fx x) >>= \case      Nothing -> return ()      Just v -> modifySTRef' l ((map (x,)  v) ++)   readSTRef l main = mapM_ print $hashJoin [(1, "Jonah"), (2, "Alan"), (3, "Glory"), (4, "Popeye")] snd [("Jonah", "Whales"), ("Jonah", "Spiders"), ("Alan", "Ghosts"), ("Alan", "Zombies"), ("Glory", "Buffy")] fst ((3,"Glory"),("Glory","Buffy")) ((2,"Alan"),("Alan","Zombies")) ((2,"Alan"),("Alan","Ghosts")) ((1,"Jonah"),("Jonah","Spiders")) ((1,"Jonah"),("Jonah","Whales"))  The task require hashtables; however, a cleaner and more functional solution would be to use Data.Map (based on binary trees): {-# LANGUAGE TupleSections #-}import qualified Data.Map as Mimport Data.Listimport Data.Maybeimport Control.Applicative mapJoin xs fx ys fy = joined where yMap = foldl' f M.empty ys f m y = M.insertWith (++) (fy y) [y] m joined = concat . mapMaybe (\x -> map (x,) <$> M.lookup (fx x) yMap) $xs main = mapM_ print$ mapJoin    [(1, "Jonah"), (2, "Alan"), (3, "Glory"), (4, "Popeye")]        snd    [("Jonah", "Whales"), ("Jonah", "Spiders"),      ("Alan", "Ghosts"), ("Alan", "Zombies"), ("Glory", "Buffy")]        fst
((1,"Jonah"),("Jonah","Spiders"))
((1,"Jonah"),("Jonah","Whales"))
((2,"Alan"),("Alan","Zombies"))
((2,"Alan"),("Alan","Ghosts"))
((3,"Glory"),("Glory","Buffy"))


## J

Data:

table1=: ;:;._2(0 :0)  27 Jonah  18 Alan  28 Glory  18 Popeye  28 Alan) table2=: ;:;._2(0 :0)  Jonah Whales  Jonah Spiders  Alan Ghosts  Alan Zombies  Glory Buffy)

The task does not specify the hash function to use, so we'll use an identity function. But SHA-1 could be used instead, with a little more work (you'd need to convert the name into the bit vector needed by the SHA-1 interface). Practically speaking, though, the only benefit of SHA-1 in this context would be to slow down the join.

Implementation:

hash=: ]dojoin=:3 :0  c1=. {.{.y  c2=. (1 {"1 y) -. a:  c3=. (2 {"1 y) -. a:  >{c1;c2;<c3) JOIN=: ; -.&a: ,/each([email protected]{."1 <@dojoin/. ]) (1 1 0&#[email protected]|."1 table1), 1 0 1#inv"1 table2

Result:

   JOIN┌─────┬──┬───────┐│Jonah│27│Whales │├─────┼──┼───────┤│Jonah│27│Spiders│├─────┼──┼───────┤│Alan │18│Ghosts │├─────┼──┼───────┤│Alan │18│Zombies│├─────┼──┼───────┤│Alan │28│Ghosts │├─────┼──┼───────┤│Alan │28│Zombies│├─────┼──┼───────┤│Glory│28│Buffy  │└─────┴──┴───────┘

## Java

Translation of: PHP
Works with: Java version 8
import java.util.*; public class HashJoin {     public static void main(String[] args) {        String[][] table1 = {{"27", "Jonah"}, {"18", "Alan"}, {"28", "Glory"},        {"18", "Popeye"}, {"28", "Alan"}};         String[][] table2 = {{"Jonah", "Whales"}, {"Jonah", "Spiders"},        {"Alan", "Ghosts"}, {"Alan", "Zombies"}, {"Glory", "Buffy"},        {"Bob", "foo"}};         hashJoin(table1, 1, table2, 0).stream()                .forEach(r -> System.out.println(Arrays.deepToString(r)));    }     static List<String[][]> hashJoin(String[][] records1, int idx1,            String[][] records2, int idx2) {         List<String[][]> result = new ArrayList<>();        Map<String, List<String[]>> map = new HashMap<>();         for (String[] record : records1) {            List<String[]> v = map.getOrDefault(record[idx1], new ArrayList<>());            v.add(record);            map.put(record[idx1], v);        }         for (String[] record : records2) {            List<String[]> lst = map.get(record[idx2]);            if (lst != null) {                lst.stream().forEach(r -> {                    result.add(new String[][]{r, record});                });            }        }         return result;    }}
[[27, Jonah], [Jonah, Whales]]
[[27, Jonah], [Jonah, Spiders]]
[[18, Alan], [Alan, Ghosts]]
[[28, Alan], [Alan, Ghosts]]
[[18, Alan], [Alan, Zombies]]
[[28, Alan], [Alan, Zombies]]
[[28, Glory], [Glory, Buffy]]

## JavaScript

### ES6

(() => {    'use strict';     // hashJoin :: [Dict] -> [Dict] -> String -> [Dict]    let hashJoin = (tblA, tblB, strJoin) => {         let [jA, jB] = strJoin.split('='),            M = tblB.reduce((a, x) => {                let id = x[jB];                return (                    a[id] ? a[id].push(x) : a[id] = [x],                    a                );            }, {});         return tblA.reduce((a, x) => {            let match = M[x[jA]];            return match ? (                a.concat(match.map(row => dictConcat(x, row)))            ) : a;        }, []);    },     // dictConcat :: Dict -> Dict -> Dict    dictConcat = (dctA, dctB) => {        let ok = Object.keys;        return ok(dctB).reduce(            (a, k) => (a['B_' + k] = dctB[k]) && a,            ok(dctA).reduce(                (a, k) => (a['A_' + k] = dctA[k]) && a, {}            )        );    };      // TEST    let lstA = [        { age: 27, name: 'Jonah' },        { age: 18, name: 'Alan' },        { age: 28, name: 'Glory' },        { age: 18, name: 'Popeye' },        { age: 28, name: 'Alan' }    ],    lstB = [        { character: 'Jonah', nemesis: 'Whales' },        { character: 'Jonah', nemesis: 'Spiders' },        { character: 'Alan', nemesis: 'Ghosts' },        { character:'Alan', nemesis: 'Zombies' },        { character: 'Glory', nemesis: 'Buffy' },        { character: 'Bob', nemesis: 'foo' }    ];     return hashJoin(lstA, lstB, 'name=character'); })();
Output:
[{"A_age":27,"A_name":"Jonah","B_character":"Jonah","B_nemesis":"Whales"},
{"A_age":27,"A_name":"Jonah","B_character":"Jonah","B_nemesis":"Spiders"},
{"A_age":18,"A_name":"Alan","B_character":"Alan","B_nemesis":"Ghosts"},
{"A_age":18,"A_name":"Alan","B_character":"Alan","B_nemesis":"Zombies"},
{"A_age":28,"A_name":"Glory","B_character":"Glory","B_nemesis":"Buffy"},
{"A_age":28,"A_name":"Alan","B_character":"Alan","B_nemesis":"Ghosts"},
{"A_age":28,"A_name":"Alan","B_character":"Alan","B_nemesis":"Zombies"}]

## jq

Works with: jq version 1.4

Relational tables can be represented in several ways in JSON, and so in this section we present two distinct "hash join" functions in jq:

• "hashJoin" can be used if the tables are represented as arrays of JSON objects, or as arrays of arrays, but the result may include the join-column twice;
• "hashJoinArrays" is intended for use if the tables are represented as arrays of arrays, and avoids the duplication mentioned above.

Both versions are relationally symmetric, and both versions allow the join columns to contain any JSON value. To achieve this generality, the collision-free hash function, h, is used.

### hashJoin

# hashJoin(table1; key1; table2; key2) expects the two tables to be# arrays, either of JSON objects, or of arrays. # In the first case, that is, if the table's rows are represented as# objects, then key1 should be the key of the join column of table1,# and similarly for key2; if the join columns have different names,# then they will both be included in the resultant objects. # In the second case, that is, if the rows are arrays, then the# 0-based indices of the join columns should be specified, and the# rows are simply pasted together, resulting in duplication of the# join columns.#def hashJoin(table1; key1; table2; key2):  # collision-free hash function:  def h:    if type == "object" then with_entries(.value = (.value|h)) | tostring    elif type == "array" then map(h)|tostring    else (type[0:1]+tostring)    end;   # hash phase:  reduce table1[] as $row ({}; ($row[key1]|h) as $key | . + { ($key): (.[$key] + [$row]) } )  | . as $hash # join phase | reduce table2[] as$row      ([];       ($row[key2]|h) as$key       | if $hash|has($key) then           reduce $hash[$key][] as $r (.; . + [$row + $r ] ) else . end); Example def table1: [ {"age": 27, "name": "Jonah"}, {"age": 18, "name": "Alan"}, {"age": 28, "name": "Glory"}, {"age": 18, "name": "Popeye"}, {"age": 28, "name": "Alan"} ]; def table2: [ {"name": "Jonah", "nemesis": "Whales"}, {"name": "Jonah", "nemesis": "Spiders"}, {"name": "Alan", "nemesis": "Ghosts"}, {"name": "Alan", "nemesis": "Zombies"}, {"name": "Glory", "nemesis": "Buffy"} ]; def table1a: [[27, "Jonah"], [18, "Alan"], [28, "Glory"], [18, "Popeye"], [28, "Alan"] ]; def table2a: [["Jonah", "Whales"], ["Jonah", "Spiders"], ["Alan", "Ghosts"], ["Alan", "Zombies"], ["Glory", "Buffy"], ["Holmes", "Moriarty"] ]; def pp: reduce .[] as$row (""; . + "\n" + ($row|tostring)); ( hashJoin(table1; "name"; table2; "name"), hashJoin(table1a; 1; table2a; 0)) | pp Output: $ jq -c -r -n -f HashJoin.jq {"age":27,"name":"Jonah","nemesis":"Whales"}{"age":27,"name":"Jonah","nemesis":"Spiders"}{"age":28,"name":"Alan","nemesis":"Ghosts"}{"age":28,"name":"Alan","nemesis":"Zombies"}{"age":28,"name":"Glory","nemesis":"Buffy"} [27,"Jonah","Jonah","Whales"][27,"Jonah","Jonah","Spiders"][28,"Alan","Alan","Ghosts"][28,"Alan","Alan","Zombies"][28,"Glory","Glory","Buffy"]

### hashJoinArrays

# The tables should be arrays of arrays;# index1 and index2 should be the 0-based indices of the join columns.#def hashJoinArrays(table1; index1; table2; index2):  # collision-free hash function:  def h:    if type == "object" then with_entries(.value = (.value|h)) | tostring    elif type == "array" then map(h)|tostring    else (type[0:1]+tostring)    end;   # hash phase:  reduce table1[] as $row ({}; ($row[index1]|h) as $key | . + (.[$key] += [ $row ]) ) | . as$hash  # join phase  | reduce table2[] as $row ([]; ($row[index2]|h) as $key | if$hash|has($key) then reduce$hash[$key][] as$r	     (.; 	      . + [ $r +$row[0:index2] + $row[index2+1:] ] ) else . end); Example In the following example, the previously defined pretty-print function (pp) and tables (table1 and table2) are used, so their definitions are not repeated here. hashJoinArrays(table1; 1; table2; 0) | pp Output: $ jq -c -r -n -f HashJoinArrays.jq [27,"Jonah","Whales"][27,"Jonah","Spiders"][28,"Alan","Ghosts"][28,"Alan","Zombies"][28,"Glory","Buffy"]

## Julia

Works with: Julia version 0.6

For dataframes there is a builtin function join:

using DataFrames A = DataFrame(Age = [27, 18, 28, 18, 28], Name = ["Jonah", "Alan", "Glory", "Popeye", "Alan"])B = DataFrame(Name = ["Jonah", "Jonah", "Alan", "Alan", "Glory"],    Nemesis = ["Whales", "Spiders", "Ghosts", "Zombies", "Buffy"])AB = join(A, B, on = :Name) @show A B AB
Output:
A = 5×2 DataFrames.DataFrame
│ Row │ Age │ Name     │
├─────┼─────┼──────────┤
│ 1   │ 27  │ "Jonah"  │
│ 2   │ 18  │ "Alan"   │
│ 3   │ 28  │ "Glory"  │
│ 4   │ 18  │ "Popeye" │
│ 5   │ 28  │ "Alan"   │
B = 5×2 DataFrames.DataFrame
│ Row │ Name    │ Nemesis   │
├─────┼─────────┼───────────┤
│ 1   │ "Jonah" │ "Whales"  │
│ 2   │ "Jonah" │ "Spiders" │
│ 3   │ "Alan"  │ "Ghosts"  │
│ 4   │ "Alan"  │ "Zombies" │
│ 5   │ "Glory" │ "Buffy"   │
AB = 7×3 DataFrames.DataFrame
│ Row │ Age │ Name    │ Nemesis   │
├─────┼─────┼─────────┼───────────┤
│ 1   │ 18  │ "Alan"  │ "Ghosts"  │
│ 2   │ 18  │ "Alan"  │ "Zombies" │
│ 3   │ 28  │ "Alan"  │ "Ghosts"  │
│ 4   │ 28  │ "Alan"  │ "Zombies" │
│ 5   │ 28  │ "Glory" │ "Buffy"   │
│ 6   │ 27  │ "Jonah" │ "Whales"  │
│ 7   │ 27  │ "Jonah" │ "Spiders" │

Following the task hint:

function hashjoin(A::Array, ja::Int, B::Array, jb::Int)    M = Dict(t[jb] => filter(l -> l[jb] == t[jb], B) for t in B)    return collect([a, b] for a in A for b in get(M, a[ja], ()))end table1 = [(27, "Jonah"),          (18, "Alan"),          (28, "Glory"),          (18, "Popeye"),          (28, "Alan")]table2 = [("Jonah", "Whales"),          ("Jonah", "Spiders"),          ("Alan", "Ghosts"),          ("Alan", "Zombies"),          ("Glory", "Buffy")] for r in hashjoin(table1, 2, table2, 1)    println(r)end
Output:
Tuple{Any,String}[(27, "Jonah"), ("Jonah", "Whales")]
Tuple{Any,String}[(27, "Jonah"), ("Jonah", "Spiders")]
Tuple{Any,String}[(18, "Alan"), ("Alan", "Ghosts")]
Tuple{Any,String}[(18, "Alan"), ("Alan", "Zombies")]
Tuple{Any,String}[(28, "Glory"), ("Glory", "Buffy")]
Tuple{Any,String}[(28, "Alan"), ("Alan", "Ghosts")]
Tuple{Any,String}[(28, "Alan"), ("Alan", "Zombies")]

## Kotlin

data class A(val age: Int, val name: String) data class B(val character: String, val nemesis: String) data class C(val rowA: A, val rowB: B) fun hashJoin(tableA: List<A>, tableB: List<B>): List<C> {    val mm = tableB.groupBy { it.character }    val tableC = mutableListOf<C>()    for (a in tableA) {        val value = mm[a.name] ?: continue        for (b in value) tableC.add(C(a, b))    }    return tableC.toList()} fun main(args: Array<String>) {    val tableA = listOf(        A(27, "Jonah"),        A(18, "Alan"),        A(28, "Glory"),        A(18, "Popeye"),        A(28, "Alan")    )    val tableB = listOf(        B("Jonah", "Whales"),        B("Jonah", "Spiders"),        B("Alan", "Ghosts"),        B("Alan", "Zombies"),        B("Glory", "Buffy")    )    val tableC = hashJoin(tableA, tableB)    println("A.Age A.Name B.Character B.Nemesis")    println("----- ------ ----------- ---------")    for (c in tableC) {        print("${c.rowA.age}${c.rowA.name.padEnd(6)} ")        println("${c.rowB.character.padEnd(6)}${c.rowB.nemesis}")    }}
Output:
A.Age A.Name B.Character B.Nemesis
----- ------ ----------- ---------
27    Jonah  Jonah       Whales
27    Jonah  Jonah       Spiders
18    Alan   Alan        Ghosts
18    Alan   Alan        Zombies
28    Glory  Glory       Buffy
28    Alan   Alan        Ghosts
28    Alan   Alan        Zombies


## LFE

 (defun hash (column table)  (lists:foldl    (lambda (x acc)      (orddict:append (proplists:get_value column x) x acc))    '()    table)) (defun get-hash (col hash-table)  (proplists:get_value     (proplists:get_value col r)     hashed)) (defun merge (row-1 row-2)  (orddict:merge    (lambda (k v1 v2) v2)    (lists:sort row-1)    (lists:sort row-2))) (defun hash-join (table-1 col-1 table-2 col-2)  (let ((hashed (hash col-1 table-1)))    (lc ((<- r table-2))        (lc ((<- s (get-hash col-2 hashed)))            (merge r s)))))

Table definitions in the LFE REPL:

 > (set ss '((#(age 27) #(name "Jonah"))            (#(age 18) #(name "Alan"))            (#(age 28) #(name "Glory"))            (#(age 18) #(name "Popeye"))            (#(age 28) #(name "Alan")))) > (set rs '((#(nemesis "Whales") #(name "Jonah"))            (#(nemesis "Spiders") #(name "Jonah"))            (#(nemesis "Ghosts") #(name "Alan"))            (#(nemesis "Zombies") #(name "Alan"))            (#(nemesis "Buffy") #(name "Glory"))))

Output in LFE REPL:

 > (hash-join ss 'name rs 'name)(((#(age 27) #(name "Jonah") #(nemesis "Whales"))) ((#(age 27) #(name "Jonah") #(nemesis "Spiders"))) ((#(age 18) #(name "Alan") #(nemesis "Ghosts"))  (#(age 28) #(name "Alan") #(nemesis "Ghosts"))) ((#(age 18) #(name "Alan") #(nemesis "Zombies"))  (#(age 28) #(name "Alan") #(nemesis "Zombies"))) ((#(age 28) #(name "Glory") #(nemesis "Buffy"))))

 Module HashJoin {      \\ normally we define variables when we put values to names      \\ so we can remove these two lines      Def Name$, Nemesis$      Def Long m, mc, items_size, A       \\ Lets make a container which use keys with hash function      Inventory A            \\ A now is a pointer to an Inventory, with Len(A)=0      \\ Print Type$(A)="Inventory" \\ empty stack. We use current stack to place data Flush \Input B data "Jonah", "Whales" data "Jonah", "Spiders" data "Alan", "Ghosts" data "Alan", "Zombies" data "Glory", "Buffy" \\ Keys are unique, This is the HASH PHASE While not empty { Read Name$, Nemesis$If Exist(A, Name$) Then {                  m=Eval(A)  ' get a pointer to array                  Stack m {Data Nemesis$} } Else Append A, Name$:=Stack:=Nemesis$' a stack object with one item } \\ Input A, this is the Long Table data 27, "Jonah" data 18, "Alan" data 28, "Glory" data 18, "Popeye" data 28, "Alan" \\ This is the JOIN PHASE items_size=stack.size/2 \\ using items_size we can append data (using data) to stack \\$(0) is the default handler for columns.      \\ Letters justify to left, numbers to right.      \\ Letters can use more columns, and maybe wrap to more lines.       Print $(0), "Output during join" Print "A.Age", "A.Name","B.Character", "B.Nemesis" While items_size>0 { Read Age, Name$            If exist(A, Name$) Then { m=Eval(A) ' extract a pointer, this is for a stack object mc=Each(m) ' make an iterator While mc { \\ we use$(1) for left justify numbers too                        \\ normal StackItem$(stackobject) return top only \\ we have to use StackItem$(stackobject, 3) to get 3rd                        \\ or StackItem(stackobject, 3) if it is numeric.                        \\ but here mc is iterator, and place the cursor value to it                       Print $(1), Age, Name$,Name$, StackItem$(mc)                        \\ so now we place at the end of current stack the same output                       Data Age, Name$,Name$, StackItem$(mc) } } items_size-- } \\ split rem line after : to use second way rem : goto secondway Print$(0), "Output after join"      Print "A.Age", "A.Name","B.Character", "B.Nemesis"      While not Empty {            Print $(1), Number, Letter$, Letter$, Letter$      }      Exitsecondway:      Print $(0), "Output after join using format$()"      Print Format$("{0:5} {1:10} {2:10} {3:20}","A.Age", "A.Name","B.Character", "B.Nemesis") While not Empty { Print format$("{0::5} {1:10} {2:10} {3:20}", Number, Letter$, Letter$, Letter$) }}HashJoin  Output: 27 Jonah Jonah Whales 27 Jonah Jonah Spiders 18 Alan Alan Ghosts 18 Alan Alan Zombies 28 Glory Glory Buffy 28 Alan Alan Ghosts 28 Alan Alan Zombies  ## Mathematica / Wolfram Language Works with: Mathematica version 10 Updated version is now able to join wider tables by giving the index. The smaller table is hashed but this might result in different column ordering. Uses Associations introduced in Mathematica Version 10 hashJoin[table1_List,table1colindex_Integer,table2_List,table2colindex_Integer]:=Module[{h,f,t1,t2,tmp},t1=If[table1colindex != 1,table1[[All,Prepend[Delete[[email protected]@table1[[1]],table1colindex],table1colindex]]],table1];t2=If[table2colindex != 1, table2[[All,Prepend[Delete[[email protected]@table2[[1]],table2colindex],table2colindex]]],table2]; If[Length[t1]>Length[t2],tmp=t1;t1=t2;t2=tmp;];h= GroupBy[t1,First];f[{a_,b_List}]:={a,#}&/@b;Partition[Flatten[Map[f,{#[[2;;]],h[#[[1]]]}&/@t2]],Length[t1[[1]]]+Length[t2[[1]]]-1]]; Sample run: table1 = {{18, "Alan", 1}, {27, "Jonah", 2}, {28, "Alan", 3}, {28, "Glory", 4}}; table2 = {{"Alan", "Ghosts"}, {"Alan", "Zombies"}, {"Glory", "Buffy"}, {"Jonah", "Spiders"}, {"Jonah", "Whales"}}; table1colindex = 2; table2colindex = 1; hashJoin[table1, table1colindex, table2, table2colindex] // TableForm Ghosts Alan 18 1 Ghosts Alan 28 3 Zombies Alan 18 1 Zombies Alan 28 3 Buffy Glory 28 4 Spiders Jonah 27 2 Whales Jonah 27 2 table1 = {{18, "Alan", 1}, {27, "Jonah", 2}, {28, "Alan", 3}, {28, "Glory", 4}}; table2 = {{33, "Alan", "Ghosts"}, {34, "Alan", "Zombies"}, {35, "Glory", "Buffy"}, {36, "Jonah", "Spiders"}, {37, "Jonah", "Whales"}}; table1colindex = 2; table2colindex = 2; hashJoin[table1, table1colindex, table2, table2colindex] // TableForm 33 Ghosts Alan 18 1 33 Ghosts Alan 28 3 34 Zombies Alan 18 1 34 Zombies Alan 28 3 35 Buffy Glory 28 4 36 Spiders Jonah 27 2 37 Whales Jonah 27 2 table1 = {{19, "Zorro", 8}, {17, "Zorro", 7}, {17, "Zorro", 9}, {18, "Alan", 1}, {27, "Jonah", 2}, {28, "Alan", 3}, {28, "Glory", 4}}; table2 = {{33, "Alan", "Ghosts"}, {34, "Alan", "Zombies"}, {35, "Glory", "Buffy"}, {36, "Jonah", "Spiders"}, {37, "Jonah", "Whales"}, {39, "Zorro", "Fox"}}; table1colindex = 2; table2colindex = 2; hashJoin[table1, table1colindex, table2, table2colindex] // TableForm 19 8 Zorro 39 Fox 17 7 Zorro 39 Fox 17 9 Zorro 39 Fox 18 1 Alan 33 Ghosts 18 1 Alan 34 Zombies 27 2 Jonah 36 Spiders 27 2 Jonah 37 Whales 28 3 Alan 33 Ghosts 28 3 Alan 34 Zombies 28 4 Glory 35 Buffy  ## Oberon-2 Works with oo2c version 2  MODULE HashJoin;IMPORT ADT:Dictionary, ADT:LinkedList, NPCT:Tools, Object, Object:Boxed, Out;TYPE (* Some Aliases *) Age= Boxed.LongInt; Name= STRING; Nemesis= STRING; (* Generic Tuple *) Tuple(E1: Object.Object; E2: Object.Object) = POINTER TO TupleDesc(E1,E2); TupleDesc(E1: Object.Object; E2: Object.Object) = RECORD (Object.ObjectDesc) _1: E1; _2: E2; END; (* Relations *) RelationA = ARRAY 5 OF Tuple(Age,Name); RelationB = ARRAY 5 OF Tuple(Name,Nemesis); VAR tableA: RelationA; tableB: RelationB; dict: Dictionary.Dictionary(Name,LinkedList.LinkedList(Age)); ll: LinkedList.LinkedList(Age); PROCEDURE (t: Tuple(E1, E2)) INIT*(e1: E1; e2: E2); BEGIN t._1 := e1; t._2 := e2; END INIT; PROCEDURE DoHashPhase(t: RelationA;VAR dict: Dictionary.Dictionary(Name,LinkedList.LinkedList(Age))); VAR i: INTEGER; ll: LinkedList.LinkedList(Age); BEGIN i := 0; WHILE (i < LEN(t)) & (t[i] # NIL) DO IF (dict.HasKey(t[i]._2)) THEN ll := dict.Get(t[i]._2); ELSE ll := NEW(LinkedList.LinkedList(Age)); dict.Set(t[i]._2,ll) END; ll.Append(t[i]._1); INC(i) END END DoHashPhase; PROCEDURE DoJoinPhase(t: RelationB; dict: Dictionary.Dictionary(Name,LinkedList.LinkedList(Age))); VAR i: INTEGER; age: Age; iterll: LinkedList.Iterator(Age); BEGIN FOR i := 0 TO LEN(t) - 1 DO ll := dict.Get(t[i]._1); iterll := ll.GetIterator(NIL); WHILE iterll.HasNext() DO age := iterll.Next(); Out.LongInt(age.value,4); Out.Object(Tools.AdjustRight(t[i]._1,10)); Out.Object(Tools.AdjustRight(t[i]._2,10));Out.Ln END END END DoJoinPhase; BEGIN (* tableA initialization *) tableA[0] := NEW(Tuple(Age,Name),NEW(Age,27),"Jonah"); tableA[1] := NEW(Tuple(Age,Name),NEW(Age,18),"Alan"); tableA[2] := NEW(Tuple(Age,Name),NEW(Age,28),"Glory"); tableA[3] := NEW(Tuple(Age,Name),NEW(Age,18),"Popeye"); tableA[4] := NEW(Tuple(Age,Name),NEW(Age,28),"Alan"); (* tableB initialization *) tableB[0] := NEW(Tuple(Name,Nemesis),"Jonah","Whales"); tableB[1] := NEW(Tuple(Name,Nemesis),"Jonah","Spiders"); tableB[2] := NEW(Tuple(Name,Nemesis),"Alan","Ghost"); tableB[3] := NEW(Tuple(Name,Nemesis),"Alan","Zombies"); tableB[4] := NEW(Tuple(Name,Nemesis),"Glory","Buffy"); dict := NEW(Dictionary.Dictionary(Name,LinkedList.LinkedList(Age))); DoHashPhase(tableA,dict); DoJoinPhase(tableB,dict);END HashJoin.  Output:  27 Jonah Whales 27 Jonah Spiders 18 Alan Ghost 28 Alan Ghost 18 Alan Zombies 28 Alan Zombies 28 Glory Buffy  ## OCaml This exploits the fact that Hashtbl implements a hash table that can store multiple values for a key, for an especially simple solution: let hash_join table1 f1 table2 f2 = let h = Hashtbl.create 42 in (* hash phase *) List.iter (fun s -> Hashtbl.add h (f1 s) s) table1; (* join phase *) List.concat (List.map (fun r -> List.map (fun s -> s, r) (Hashtbl.find_all h (f2 r))) table2) Sample run: # let table1 = [27, "Jonah"; 18, "Alan"; 28, "Glory"; 18, "Popeye"; 28, "Alan"];; # let table2 = ["Jonah", "Whales"; "Jonah", "Spiders"; "Alan", "Ghosts"; "Alan", "Zombies"; "Glory", "Buffy"];; # hash_join table1 snd table2 fst;; - : ((int * string) * (string * string)) list = [((27, "Jonah"), ("Jonah", "Whales")); ((27, "Jonah"), ("Jonah", "Spiders")); ((28, "Alan"), ("Alan", "Ghosts")); ((18, "Alan"), ("Alan", "Ghosts")); ((28, "Alan"), ("Alan", "Zombies")); ((18, "Alan"), ("Alan", "Zombies")); ((28, "Glory"), ("Glory", "Buffy"))]  ## Perl use Data::Dumper qw(Dumper); sub hashJoin { my ($table1, $index1,$table2, $index2) = @_; my %h; # hash phase foreach my$s (@$table1) { push @{$h{$s->[$index1]} }, $s; } # join phase map { my$r = $_; map [$_, $r], @{$h{$r->[$index2]} }    } @$table2;} @table1 = ([27, "Jonah"], [18, "Alan"], [28, "Glory"], [18, "Popeye"], [28, "Alan"]);@table2 = (["Jonah", "Whales"], ["Jonah", "Spiders"], ["Alan", "Ghosts"], ["Alan", "Zombies"], ["Glory", "Buffy"]);$Data::Dumper::Indent = 0;foreach my $row (hashJoin(\@table1, 1, \@table2, 0)) { print Dumper($row), "\n";}
Output:
$VAR1 = [[27,'Jonah'],['Jonah','Whales']];$VAR1 = [[27,'Jonah'],['Jonah','Spiders']];
$VAR1 = [[18,'Alan'],['Alan','Ghosts']];$VAR1 = [[28,'Alan'],['Alan','Ghosts']];
$VAR1 = [[18,'Alan'],['Alan','Zombies']];$VAR1 = [[28,'Alan'],['Alan','Zombies']];
$VAR1 = [[28,'Glory'],['Glory','Buffy']];  ## Perl 6 The .classify method returns a multimap represented as a Hash whose values are Arrays. Works with: Rakudo version 2016.07 sub hash-join(@a, &a, @b, &b) { my %hash := @b.classify(&b); @a.map: ->$a {        |(%hash{a $a} // next).map: ->$b { [$a,$b] }    }} # Testing: my @A =    [27, "Jonah"],    [18, "Alan"],    [28, "Glory"],    [18, "Popeye"],    [28, "Alan"],; my @B =    ["Jonah", "Whales"],    ["Jonah", "Spiders"],    ["Alan", "Ghosts"],    ["Alan", "Zombies"],    ["Glory", "Buffy"],; .say for hash-join @A, *[1], @B, *[0];
Output:
[[27 Jonah] [Jonah Whales]]
[[27 Jonah] [Jonah Spiders]]
[[18 Alan] [Alan Ghosts]]
[[18 Alan] [Alan Zombies]]
[[28 Glory] [Glory Buffy]]
[[28 Alan] [Alan Ghosts]]
[[28 Alan] [Alan Zombies]]

## Phix

Phix dictionary keys must be unique, but storing/extending a sequence is no trouble, and in fact simplifies the scan phase.

constant A = {{27,"Jonah"},              {18,"Alan"},              {28,"Glory"},              {18,"Popeye"},              {28,"Alan"}},         B = {{"Jonah","Whales"},              {"Jonah","Spiders"},              {"Alan", "Ghosts"},              {"Alan", "Zombies"},              {"Glory","Buffy"}},        jA = 2,        jB = 1,        MB = new_dict()sequence C = {}for i=1 to length(B) do    object key = B[i][jB]    object data = getd(key,MB)    if data=0 then        data = {B[i]}    else        data = append(data,B[i])    end if    putd(key,data,MB)end forfor i=1 to length(A) do    object data = getd(A[i][jA],MB)    if sequence(data) then        for j=1 to length(data) do            C = append(C,{A[i],data[j]})        end for    end ifend fordestroy_dict(MB)pp(C,{pp_Nest,1})
Output:
{{{27, "Jonah"}, {"Jonah", "Whales"}},
{{27, "Jonah"}, {"Jonah", "Spiders"}},
{{18, "Alan"}, {"Alan", "Ghosts"}},
{{18, "Alan"}, {"Alan", "Zombies"}},
{{28, "Glory"}, {"Glory", "Buffy"}},
{{28, "Alan"}, {"Alan", "Ghosts"}},
{{28, "Alan"}, {"Alan", "Zombies"}}}


## PHP

<?phpfunction hashJoin($table1,$index1, $table2,$index2) {    // hash phase    foreach ($table1 as$s)        $h[$s[$index1]][] =$s;    // join phase    foreach ($table2 as$r)    	foreach ($h[$r[$index2]] as$s)	    $result[] = array($s, $r); return$result;} $table1 = array(array(27, "Jonah"), array(18, "Alan"), array(28, "Glory"), array(18, "Popeye"), array(28, "Alan"));$table2 = array(array("Jonah", "Whales"),           array("Jonah", "Spiders"),           array("Alan", "Ghosts"),           array("Alan", "Zombies"),           array("Glory", "Buffy"),           array("Bob", "foo")); foreach (hashJoin($table1, 1,$table2, 0) as $row) print_r($row);?>
Output:
Array
(
[0] => Array
(
[0] => 27
[1] => Jonah
)

[1] => Array
(
[0] => Jonah
[1] => Whales
)

)
Array
(
[0] => Array
(
[0] => 27
[1] => Jonah
)

[1] => Array
(
[0] => Jonah
[1] => Spiders
)

)
Array
(
[0] => Array
(
[0] => 18
[1] => Alan
)

[1] => Array
(
[0] => Alan
[1] => Ghosts
)

)
Array
(
[0] => Array
(
[0] => 28
[1] => Alan
)

[1] => Array
(
[0] => Alan
[1] => Ghosts
)

)
Array
(
[0] => Array
(
[0] => 18
[1] => Alan
)

[1] => Array
(
[0] => Alan
[1] => Zombies
)

)
Array
(
[0] => Array
(
[0] => 28
[1] => Alan
)

[1] => Array
(
[0] => Alan
[1] => Zombies
)

)
Array
(
[0] => Array
(
[0] => 28
[1] => Glory
)

[1] => Array
(
[0] => Glory
[1] => Buffy
)

)

## PicoLisp

(de A   (27 . Jonah)   (18 . Alan)   (28 . Glory)   (18 . Popeye)   (28 . Alan) ) (de B   (Jonah . Whales)   (Jonah . Spiders)   (Alan . Ghosts)   (Alan . Zombies)   (Glory . Buffy) ) (for X B   (let K (cons (char (hash (car X))) (car X))      (if (idx 'M K T)         (push (caar @) (cdr X))         (set (car K) (list (cdr X))) ) ) ) (for X A   (let? Y (car (idx 'M (cons (char (hash (cdr X))) (cdr X))))      (for Z (caar Y)         (println (car X) (cdr X) (cdr Y) Z) ) ) )

Output:

27 Jonah Jonah Spiders
27 Jonah Jonah Whales
18 Alan Alan Zombies
18 Alan Alan Ghosts
28 Glory Glory Buffy
28 Alan Alan Zombies
28 Alan Alan Ghosts

## Plain TeX

Works with any TeX engine.

\newtoks\tabjoin\def\quark{\quark}\def\tabA{27:Jonah,18:Alan,28:Glory,18:Popeye,28:Alan}\def\tabB{Jonah:Whales,Jonah:Spiders,Alan:Ghosts,Alan:Zombies,Glory:Buffy}\def\mergejoin{\tabjoin{}\expandafter\mergejoini\tabA,\quark:\quark,}\def\mergejoini#1:#2,{%	\ifx\quark#1\the\tabjoin	\else		\def\mergejoinii##1,#2:##2,{%			\ifx\quark##2\else				\tabjoin\expandafter{\the\tabjoin#1 : #2 : ##2\par}%				\expandafter\mergejoinii\expandafter,%			\fi		}%		\expandafter\mergejoinii\expandafter,\tabB,#2:\quark,%		\expandafter\mergejoini	\fi}\mergejoin\bye

pdf or dvi output:

27 : Jonah : Whales
27 : Jonah : Spiders
18 : Alan : Ghosts
18 : Alan : Zombies
28 : Glory : Buffy
28 : Alan : Ghosts
28 : Alan : Zombies

## PureBasic

Structure tabA  age.i  name.s  EndStructure Structure tabB  char_name.s  nemesis.sEndStructure NewList listA.tabA()NewList listB.tabB() Macro SetListA(c_age, c_name)  AddElement(listA()) : listA()\age = c_age : listA()\name = c_name  EndMacro Macro SetListB(c_char, c_nem)  AddElement(listB()) : listB()\char_name = c_char : listB()\nemesis = c_nemEndMacro SetListA(27, "Jonah")   : SetListA(18, "Alan") : SetListA(28, "Glory")SetListA(18, "Popeye")  : SetListA(28, "Alan") SetListB("Jonah", "Whales") : SetListB("Jonah", "Spiders")SetListB("Alan", "Ghosts")  : SetListB("Alan", "Zombies")SetListB("Glory", "Buffy") If OpenConsole("Hash_join")  ForEach listA()    PrintN("Input A = "+Str(listA()\age)+~"\t"+listA()\name)  Next  PrintN("")  ForEach listB()    PrintN("Input B = "+listB()\char_name+~"\t"+listB()\nemesis)  Next  PrintN(~"\nOutput\nA.Age\tA.Name\tB.Char.\tB.Nemesis")  ForEach listA()    ForEach listB()      If listA()\name = listB()\char_name        PrintN(Str(listA()\age)+~"\t"+listA()\name+~"\t"+               listB()\char_name+~"\t"+listB()\nemesis)      EndIf    Next  Next  Input()EndIf
Output:
Input A = 27    Jonah
Input A = 18    Alan
Input A = 28    Glory
Input A = 18    Popeye
Input A = 28    Alan

Input B = Jonah Whales
Input B = Jonah Spiders
Input B = Alan  Ghosts
Input B = Alan  Zombies
Input B = Glory Buffy

Output
A.Age   A.Name  B.Char. B.Nemesis
27      Jonah   Jonah   Whales
27      Jonah   Jonah   Spiders
18      Alan    Alan    Ghosts
18      Alan    Alan    Zombies
28      Glory   Glory   Buffy
28      Alan    Alan    Ghosts
28      Alan    Alan    Zombies

## Python

from collections import defaultdict def hashJoin(table1, index1, table2, index2):    h = defaultdict(list)    # hash phase    for s in table1:        h[s[index1]].append(s)    # join phase    return [(s, r) for r in table2 for s in h[r[index2]]] table1 = [(27, "Jonah"),          (18, "Alan"),          (28, "Glory"),          (18, "Popeye"),          (28, "Alan")]table2 = [("Jonah", "Whales"),          ("Jonah", "Spiders"),          ("Alan", "Ghosts"),          ("Alan", "Zombies"),          ("Glory", "Buffy")] for row in hashJoin(table1, 1, table2, 0):    print(row)
Output:
((27, 'Jonah'), ('Jonah', 'Whales'))
((27, 'Jonah'), ('Jonah', 'Spiders'))
((18, 'Alan'), ('Alan', 'Ghosts'))
((28, 'Alan'), ('Alan', 'Ghosts'))
((18, 'Alan'), ('Alan', 'Zombies'))
((28, 'Alan'), ('Alan', 'Zombies'))
((28, 'Glory'), ('Glory', 'Buffy'))


## Racket

#lang racket(struct A  (age name))(struct B  (name nemesis))(struct AB (name age nemesis) #:transparent) (define Ages-table  (list (A 27 "Jonah") (A 18 "Alan")        (A 28 "Glory") (A 18 "Popeye")        (A 28 "Alan"))) (define Nemeses-table  (list   (B "Jonah" "Whales") (B "Jonah" "Spiders")   (B "Alan" "Ghosts") (B "Alan" "Zombies")   (B "Glory" "Buffy"))) ;; Hash phase(define name->ages#  (for/fold ((rv (hash)))    ((a (in-list Ages-table)))    (match-define (A age name) a)    (hash-update rv name (λ (ages) (append ages (list age))) null))) ;; Join phase     (for*/list    ((b (in-list Nemeses-table))     (key (in-value (B-name b)))     (age (in-list (hash-ref name->ages# key))))  (AB key age (B-nemesis b)))
Output:
(#(struct:AB "Jonah" 27 "Whales")
#(struct:AB "Jonah" 27 "Spiders")
#(struct:AB "Alan" 18 "Ghosts")
#(struct:AB "Alan" 28 "Ghosts")
#(struct:AB "Alan" 18 "Zombies")
#(struct:AB "Alan" 28 "Zombies")
#(struct:AB "Glory" 28 "Buffy"))

## REXX

/*REXX program demonstrates the  classic hash join algorithm  for two relations.        */                     S.  =                   ;         R.  =                     S.1 = 27 'Jonah'        ;         R.1 = "Jonah Whales"                     S.2 = 18 'Alan'         ;         R.2 = "Jonah Spiders"                     S.3 = 28 'Glory'        ;         R.3 = "Alan Ghosts"                     S.4 = 18 'Popeye'       ;         R.4 = "Alan Zombies"                     S.5 = 28 'Alan'         ;         R.5 = "Glory Buffy"hash.=                                           /*initialize the  hash  table (array). */      do #=1  while S.#\=='';   parse var  S.#  age  name          /*extract information*/      hash.name=hash.name #                      /*build a hash table entry with its idx*/      end   /*#*/                                /* [↑]  REXX does the heavy work here. */#=#-1                                            /*adjust for the DO loop  (#)  overage.*/      do j=1  while R.j\==''                     /*process a nemesis for a name element.*/      parse var  R.j  x  nemesis                 /*extract the name  and  its nemesis.  */      if hash.x==''  then do;   #=# + 1          /*Not in hash?  Then a new name; bump #*/                                S.#=','  x       /*add a new name to the    S   table.  */                                hash.x=#         /* "  "  "    "   "  "   hash    "     */                          end                    /* [↑]  this  DO  isn't used today.    */           do k=1  for words(hash.x);   _=word(hash.x, k)          /*obtain the pointer.*/           S._=S._  nemesis                      /*add the nemesis ──► applicable hash. */           end   /*k*/      end        /*j*/_='─'                                            /*the character used for the separator.*/pad=left('', 4)                                  /*spacing used in header and the output*/say  pad  center('age', 3)   pad   center("name", 20   )   pad    center('nemesis', 30   )say  pad  center('───', 3)   pad   center(""    , 20, _)   pad    center(''       , 30, _)       do n=1  for #;      parse  var  S.n    age  name  nems       /*obtain information.*/      if nems==''  then iterate                                    /*No nemesis?  Skip. */      say pad right(age,3) pad center(name,20) pad center(nems,30) /*display an  "S".   */      end   /*n*/                                /*stick a fork in it,  we're all done. */

output   when using the in-code relations (data):

     age              name                         nemesis
───      ────────────────────      ──────────────────────────────
27             Jonah                      Whales Spiders
18              Alan                      Ghosts Zombies
28             Glory                          Buffy
28              Alan                      Ghosts Zombies


## Ring

Table1 = [[27, "Jonah"], [18, "Alan"], [28, "Glory"], [18, "Popeye"], [28, "Alan"]]Table2 = [["Jonah", "Whales"], ["Jonah", "Spiders"], ["Alan", "Ghosts"], ["Alan", "Zombies"], ["Glory", "Buffy"]]hTable = []Qtable = [] for a in table1	h = hashing(a[2])	add(htable,[h , a])next for b in table2	h = hashing(b[1])	for sh in htable		if sh[1] = h 			 add(qtable, sh[2] + b[2])  		ok	nextnext print(qtable) #===============End of Execution========= func print lstsee "---------------------------------------------------Age	| Name		|| Name		| Nemesis---------------------------------------------------"for l in lst	see string(l[1]) + char(9) + "| " + l[2] + copy(char(9),2) + "|| " + l[2] + "    " + char(9) +  "| " + l[3] + nl next  func Hashing strr = 0if len(str) > 4	r = (ascii(str[1]) + ascii(str[len(str)]) + ascii(str[ceil(len(str) * 0.25)]) + ascii(str[ceil(len(str) * 0.75)])) else	for s in str 		r += ascii(s) 	nextokreturn r
Output:
---------------------------------------------------
Age     | Name          || Name         | Nemesis
---------------------------------------------------
27      | Jonah         || Jonah        | Whales
27      | Jonah         || Jonah        | Whales
18      | Alan          || Alan         | Ghosts
28      | Alan          || Alan         | Ghosts
18      | Alan          || Alan         | Ghosts
28      | Alan          || Alan         | Ghosts
28      | Glory         || Glory        | Buffy


## Ruby

def hashJoin(table1, index1, table2, index2)  # hash phase  h = table1.group_by {|s| s[index1]}  h.default = []  # join phase  table2.collect {|r|    h[r[index2]].collect {|s| [s, r]}  }.flatten(1)end table1 = [[27, "Jonah"],          [18, "Alan"],          [28, "Glory"],          [18, "Popeye"],          [28, "Alan"]]table2 = [["Jonah", "Whales"],          ["Jonah", "Spiders"],          ["Alan", "Ghosts"],          ["Alan", "Zombies"],          ["Glory", "Buffy"]] hashJoin(table1, 1, table2, 0).each { |row| p row }
Output:
[[27, "Jonah"], ["Jonah", "Whales"]]
[[27, "Jonah"], ["Jonah", "Spiders"]]
[[18, "Alan"], ["Alan", "Ghosts"]]
[[28, "Alan"], ["Alan", "Ghosts"]]
[[18, "Alan"], ["Alan", "Zombies"]]
[[28, "Alan"], ["Alan", "Zombies"]]
[[28, "Glory"], ["Glory", "Buffy"]]


## Run BASIC

sqliteconnect #mem, ":memory:" #mem execute("CREATE TABLE t_age(age,name)")#mem execute("CREATE TABLE t_name(name,nemesis)") #mem execute("INSERT INTO t_age VALUES(27,'Jonah')")#mem execute("INSERT INTO t_age VALUES(18,'Alan')")#mem execute("INSERT INTO t_age VALUES(28,'Glory')")#mem execute("INSERT INTO t_age VALUES(18,'Popeye')")#mem execute("INSERT INTO t_age VALUES(28,'Alan')") #mem execute("INSERT INTO t_name VALUES('Jonah','Whales')")#mem execute("INSERT INTO t_name VALUES('Jonah','Spiders')")#mem execute("INSERT INTO t_name VALUES('Alan','Ghosts')")#mem execute("INSERT INTO t_name VALUES('Alan','Zombies')")#mem execute("INSERT INTO t_name VALUES('Glory','Buffy')") #mem execute("SELECT *,t_age.name FROM t_age LEFT JOIN t_name ON t_name.name = t_age.name")WHILE  #mem hasanswer()	#row		= #mem #nextrow()	age		= #row age()	name$= #row name$()	nemesis$= #row nemesis$()print age;" ";name$;" ";nemesis$WEND
Output:
27 Jonah Spiders
27 Jonah Whales
18 Alan Ghosts
18 Alan Zombies
28 Glory Buffy
18 Popeye
28 Alan Ghosts
28 Alan Zombies

## Rust

use std::collections::HashMap;use std::hash::Hash; // If you know one of the tables is smaller, it is best to make it the second parameter.fn hash_join<A, B, K>(first: &[(K, A)], second: &[(K, B)]) -> Vec<(A, K, B)>where    K: Hash + Eq + Copy,    A: Copy,    B: Copy,{    let mut hash_map = HashMap::new();     // hash phase    for &(key, val_a) in second {        // collect all values by their keys, appending new ones to each existing entry        hash_map.entry(key).or_insert_with(Vec::new).push(val_a);    }     let mut result = Vec::new();    // join phase    for &(key, val_b) in first {        if let Some(vals) = hash_map.get(&key) {            let tuples = vals.iter().map(|&val_a| (val_b, key, val_a));            result.extend(tuples);        }    }     result} fn main() {    let table1 = [("Jonah", 27), ("Alan", 18), ("Glory", 28), ("Popeye", 18), ("Alan", 28)];    let table2 = [        ("Jonah", "Whales"), ("Jonah", "Spiders"), ("Alan", "Ghosts"),        ("Alan", "Zombies"), ("Glory", "Buffy")    ];    let result = hash_join(&table1, &table2);    println!("Age | Character Name | Nemesis");    println!("----|----------------|--------");    for (age, name, nemesis) in result {        println!("{:<3} | {:^14} | {}", age, name, nemesis);    }}
Output:
Age | Character Name | Nemesis
----|----------------|--------
27  |     Jonah      | Whales
27  |     Jonah      | Spiders
18  |      Alan      | Ghosts
18  |      Alan      | Zombies
28  |     Glory      | Buffy
28  |      Alan      | Ghosts
28  |      Alan      | Zombies

## Scala

def join[Type](left: Seq[Seq[Type]], right: Seq[Seq[Type]]) = {    val hash = right.groupBy(_.head) withDefaultValue Seq()    left.flatMap(cols => hash(cols.last).map(cols ++ _.tail))} // Example: val table1 = List(List("27", "Jonah"),                  List("18", "Alan"),                  List("28", "Glory"),                  List("18", "Popeye"),                  List("28", "Alan"))val table2 = List(List("Jonah", "Whales"),                  List("Jonah", "Spiders"),                  List("Alan", "Ghosts"),                  List("Alan", "Zombies"),                  List("Glory", "Buffy")) println(join(table1, table2) mkString "\n")
Output:
List(27, Jonah, Whales)
List(27, Jonah, Spiders)
List(18, Alan, Ghosts)
List(18, Alan, Zombies)
List(28, Glory, Buffy)
List(28, Alan, Ghosts)
List(28, Alan, Zombies)

## Scheme

Works with: Gauche Scheme
(use srfi-42) (define ages '((27 Jonah) (18 Alan) (28 Glory) (18 Popeye) (28 Alan))) (define nemeses '((Jonah Whales) (Jonah Spiders) (Alan Ghosts)                  (Alan Zombies) (Glory Buffy)                  (unknown nothing))) (define hash (make-hash-table 'equal?)) (dolist (item ages)  (hash-table-push! hash (last item) (car item))) (do-ec  (: person nemeses)  (:let name (car person))  (if (hash-table-exists? hash name))  (: age (~ hash name))  (print (list (list age name)               person)))
Output:
((27 Jonah) (Jonah Whales))
((27 Jonah) (Jonah Spiders))
((28 Alan) (Alan Ghosts))
((18 Alan) (Alan Ghosts))
((28 Alan) (Alan Zombies))
((18 Alan) (Alan Zombies))
((28 Glory) (Glory Buffy))


## Sidef

func hashJoin(table1, index1, table2, index2) {    var a = []    var h = Hash()     # hash phase    table1.each { |s|        h{s[index1]} := [] << s    }     # join phase    table2.each { |r|        a += h{r[index2]}.map{[_,r]}    }     return a} var t1  = [[27, "Jonah"],           [18, "Alan"],           [28, "Glory"],           [18, "Popeye"],           [28, "Alan"]] var t2  = [["Jonah", "Whales"],           ["Jonah", "Spiders"],           ["Alan", "Ghosts"],           ["Alan", "Zombies"],           ["Glory", "Buffy"]] hashJoin(t1, 1, t2, 0).each { .say }
Output:
[[27, 'Jonah'], ['Jonah', 'Whales']]
[[27, 'Jonah'], ['Jonah', 'Spiders']]
[[18, 'Alan'], ['Alan', 'Ghosts']]
[[28, 'Alan'], ['Alan', 'Ghosts']]
[[18, 'Alan'], ['Alan', 'Zombies']]
[[28, 'Alan'], ['Alan', 'Zombies']]
[[28, 'Glory'], ['Glory', 'Buffy']]

## SQL

Setting up the data is a bit verbose:

CREATE TABLE people (age DECIMAL(3), name VARCHAR(16));INSERT INTO people (age, name) VALUES (27, 'Jonah');INSERT INTO people (age, name) VALUES (18, 'Alan');INSERT INTO people (age, name) VALUES (28, 'Glory');INSERT INTO people (age, name) VALUES (18, 'Popeye');INSERT INTO people (age, name) VALUES (28, 'Alan'); CREATE TABLE nemesises (name VARCHAR(16), nemesis VARCHAR(16));INSERT INTO nemesises (name, nemesis) VALUES ('Jonah', 'Whales');INSERT INTO nemesises (name, nemesis) VALUES ('Jonah', 'Spiders');INSERT INTO nemesises (name, nemesis) VALUES ('Alan', 'Ghosts');INSERT INTO nemesises (name, nemesis) VALUES ('Alan', 'Zombies');INSERT INTO nemesises (name, nemesis) VALUES ('Glory', 'Buffy');

Doing the join is concise. But we don't actually have control over how the join is implemented, so this might not actually be a hash join...

SELECT * FROM people p JOIN nemesises n ON p.name = n.name
Output:
       AGE NAME             NAME             NEMESIS
---------- ---------------- ---------------- ----------------
27 Jonah            Jonah            Whales
27 Jonah            Jonah            Spiders
28 Alan             Alan             Ghosts
18 Alan             Alan             Ghosts
28 Alan             Alan             Zombies
18 Alan             Alan             Zombies
28 Glory            Glory            Buffy

## Tcl

Tcl uses hash tables to implement both its associative arrays and its dictionaries.

package require Tcl 8.6# Only for lmap, which can be replaced with foreach proc joinTables {tableA a tableB b} {    # Optimisation: if the first table is longer, do in reverse order    if {[llength $tableB] < [llength$tableA]} {	return [lmap pair [joinTables $tableB$b $tableA$a] {	    lreverse $pair }] } foreach value$tableA {	lappend hashmap([lindex $value$a]) [lreplace $value$a $a] #dict version# dict lappend hashmap [lindex$value $a] [lreplace$value $a$a]    }    set result {}    foreach value $tableB { set key [lindex$value $b] if {![info exists hashmap($key)]} continue	#dict version# if {![dict exists $hashmap$key]} continue	foreach first $hashmap($key) {	    #dict version# foreach first [dict get $hashmap$key]	    lappend result [list {*}$first$key {*}[lreplace $value$b $b]] } } return$result} set tableA {    {27 "Jonah"} {18 "Alan"} {28 "Glory"} {18 "Popeye"} {28 "Alan"}}set tableB {    {"Jonah" "Whales"} {"Jonah" "Spiders"} {"Alan" "Ghosts"} {"Alan" "Zombies"}    {"Glory" "Buffy"}}set joined [joinTables $tableA 1$tableB 0]foreach row $joined { puts$row}
Output:
27 Jonah Whales
27 Jonah Spiders
18 Alan Ghosts
28 Alan Ghosts
18 Alan Zombies
28 Alan Zombies
28 Glory Buffy


## TXR

Generic hash join. Arguments left-key and right-key are functions applied to the elements of the left and right sequences to retrieve the join key.

(defvar age-name '((27 Jonah)                   (18 Alan)                   (28 Glory)                   (18 Popeye)                   (28 Alan))) (defvar nemesis-name '((Jonah Whales)                       (Jonah Spiders)                       (Alan Ghosts)                       (Alan Zombies)                       (Glory Buffy))) (defun hash-join (left left-key right right-key)  (let ((join-hash [group-by left-key left])) ;; hash phase    (append-each ((r-entry right))            ;; join phase      (collect-each ((l-entry [join-hash [right-key r-entry]]))        ^(,l-entry ,r-entry))))) (format t "~s\n" [hash-join age-name second nemesis-name first])
Output:
\$ txr hash-join.tl
(((27 Jonah) (Jonah Whales)) ((27 Jonah) (Jonah Spiders)) ((18 Alan) (Alan Ghosts)) ((28 Alan) (Alan Ghosts)) ((18 Alan) (Alan Zombies)) ((28 Alan) (Alan Zombies)) ((28 Glory) (Glory Buffy)))

## VBScript

 Dim t_age(4,1)t_age(0,0) = 27 : t_age(0,1) = "Jonah"t_age(1,0) = 18 : t_age(1,1) = "Alan"t_age(2,0) = 28 : t_age(2,1) = "Glory"t_age(3,0) = 18 : t_age(3,1) = "Popeye"t_age(4,0) = 28 : t_age(4,1) = "Alan" Dim t_nemesis(4,1)t_nemesis(0,0) = "Jonah" : t_nemesis(0,1) = "Whales"t_nemesis(1,0) = "Jonah" : t_nemesis(1,1) = "Spiders"t_nemesis(2,0) = "Alan" : t_nemesis(2,1) = "Ghosts"t_nemesis(3,0) = "Alan" : t_nemesis(3,1) = "Zombies"t_nemesis(4,0) = "Glory" : t_nemesis(4,1) = "Buffy" Call hash_join(t_age,1,t_nemesis,0) Sub hash_join(table_1,index_1,table_2,index_2)	Set hash = CreateObject("Scripting.Dictionary")	For i = 0 To UBound(table_1)		hash.Add i,Array(table_1(i,0),table_1(i,1))	Next	For j = 0 To UBound(table_2)		For Each key In hash.Keys			If hash(key)(index_1) = table_2(j,index_2) Then				WScript.StdOut.WriteLine hash(key)(0) & "," & hash(key)(1) &_					" = " & table_2(j,0) & "," & table_2(j,1)			End If		Next	NextEnd Sub
Output:
27,Jonah = Jonah,Whales
27,Jonah = Jonah,Spiders
18,Alan = Alan,Ghosts
28,Alan = Alan,Ghosts
18,Alan = Alan,Zombies
28,Alan = Alan,Zombies
28,Glory = Glory,Buffy


## Visual FoxPro

Hashing using the common key (name) gives ambiguous results as the name column is not unique in either table (a unique key could be formed by using the age and name columns) . This implementation forces a unique key on the people table.

 LOCAL i As Integer, n As IntegerCLOSE DATABASES ALL*!* Create and populate the hash tablesCREATE CURSOR people_ids(id I, used L DEFAULT .F.)INDEX ON id TAG id COLLATE "Machine"INDEX ON used TAG used BINARY COLLATE "Machine"SET ORDER TO 0CREATE CURSOR nem_ids(id I, used L DEFAULT .F.)INDEX ON id TAG id COLLATE "Machine"INDEX ON used TAG used BINARY COLLATE "Machine"SET ORDER TO 0n = 100FOR i = 1 TO n    INSERT INTO people_ids (id) VALUES (i)    INSERT INTO nem_ids (id) VALUES (i)ENDFOR	 CREATE CURSOR people (age I, name V(16), id I)INDEX ON id TAG id COLLATE "Machine"INDEX ON name TAG name COLLATE "Machine"SET ORDER TO 0INSERT INTO people (age, name) VALUES (27, "Jonah")INSERT INTO people (age, name) VALUES (18, "Alan")INSERT INTO people (age, name) VALUES (28, "Glory")INSERT INTO people (age, name) VALUES (18, "Popeye")INSERT INTO people (age, name) VALUES (28, "Alan")REPLACE id WITH HashMe("people_ids") ALL *!* The plural of nemesis is nemesesCREATE CURSOR nemeses (name V(16), nemesis V(16), p_id I, id I)INDEX ON id TAG id COLLATE "Machine"INDEX ON p_id TAG p_id COLLATE "Machine"INDEX ON name TAG name COLLATE "Machine"SET ORDER TO 0 INSERT INTO nemeses (name, nemesis) VALUES ("Jonah", "Whales")INSERT INTO nemeses (name, nemesis) VALUES ("Jonah", "Spiders")INSERT INTO nemeses (name, nemesis) VALUES ("Alan", "Ghosts")INSERT INTO nemeses (name, nemesis) VALUES ("Alan", "Zombies")INSERT INTO nemeses (name, nemesis) VALUES ("Glory", "Buffy")REPLACE id WITH HashMe("nem_ids") ALL UPDATE nemeses SET p_id = people.id FROM people ;WHERE nemeses.name = people.name *!* Show the joinSELECT pe.age, pe.name, ne.nemesis FROM people pe ;JOIN nemeses ne ON pe.id = ne.p_id TO FILE "hashjoin.txt" FUNCTION HashMe(cTable As String) As IntegerLOCAL ARRAY a[1]SELECT MIN(id) FROM (cTable) WHERE NOT used INTO ARRAY aUPDATE (cTable) SET used = .T. WHERE id = a[1]RETURN a[1]ENDFUNC
Output:
AGE NAME      NEMESIS
27 Jonah     Whales
27 Jonah     Spiders
18 Alan      Ghosts
18 Alan      Zombies
28 Glory     Buffy


## zkl

Join two tables by hashing on the common key (name). The resulting join is the intersection of the two tables.

ageName:=T(27,"Jonah", 18,"Alan", 28,"Glory", 18,"Popeye", 28,"Alan");nameNemesis:=T("Jonah","Whales", "Jonah","Spiders", "Alan","Ghosts",       "Alan","Zombies", "Glory","Buffy"); fcn addAN(age,name,d){  // keys are names, values are ( (age,...),() )   if (r:=d.find(name)) d[name] = T(r[0].append(age),r[1]);   else d.add(name,T(T(age),T));   d	// return d so pump will use that as result for assignment}fcn addNN(name,nemesis,d){ // values-->( (age,age...), (nemesis,...) )   if (r:=d.find(name)){      ages,nemesises := r;      d[name] = T(ages,nemesises.append(nemesis));   }}    // Void.Read --> take existing i, read next one, pass both to next functionvar d=ageName.pump(Void,Void.Read,T(addAN,Dictionary()));nameNemesis.pump(Void,Void.Read,T(addNN,d)); d.println();  // the union of the two tablesd.keys.sort().pump(Console.println,'wrap(name){  //pretty print the join   val:=d[name]; if (not val[1])return(Void.Skip);   String(name,":",d[name][1].concat(","));})

zkl Dictionaries only have one key

D(Popeye:L(L(18),L()),Glory:L(L(28),L("Buffy")),
Jonah:L(L(27),L("Whales","Spiders")),Alan:L(L(18,28),L("Ghosts","Zombies")))
Alan:Ghosts,Zombies
Glory:Buffy
Jonah:Whales,Spiders