Hash join: Difference between revisions

</pre>

=={{header|OCaml}}==

This exploits the fact that Hashtbl implements a hash table that can store multiple values for a key, for an especially simple solution:

<lang ocaml>let hash_join table1 table2 =

let h = Hashtbl.create 42 in

(* hash phase *)

List.iter (fun (_,k as s) ->

Hashtbl.add h k s) table1;

(* join phase *)

List.concat (List.map (fun (k,_ as r) ->

List.map (fun s -> s, r) (Hashtbl.find_all h k)) table2)</lang>

Sample run:

<pre>

# 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 table2;;

- : ((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"))]