How to express LEFT JOIN

[tkawachi]

I think a Conjunction is equivalent to an INNER JOIN in SQL, but how do I describe a LEFT JOIN?

[EvgSkv]

Left join is done via a combine expression. Like this:

JoinedTable(key:, some_value:, joined_value:) :-
  MainTable(key:, some_value:),
  joined_value AnyValue= (x :- OuterTable(key:, value: x));

AnyValue should be used if it is known that OuterTable has no more than one row with the given key.
If it could have many values that List can be used to collect all of them, with appropriate further processing.

I have also added an example in the scripts directory of assembling contacts with the outer join.

Please let me know if you have any questions or comments about this.

[tkawachi]

Thanks, I'll try it out. 😄