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Left JoinAcross when right set is empty

Posted 8 days ago
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I have a question about JoinAcross. In this case the left association is returned unchanged:

JoinAcross[{<|a -> 1, b -> X|>}, {<|a -> 2|>}, Key[a], "Left"]
(* {<|a -> 1, b -> X|>} *)

The result is expected. Notice that the "Left" is documented as allowing the first set (left) for which there is no matching right-side set. I was surprised to find that the following code return an empty list:

JoinAcross[{<|a -> 1, b -> X|>}, {}, Key[a], "Left"]
(* { } *)

Is this the expected behavior?

I now need to change my code so that if the right-side set is empty, then I need to insert a dummy association to prevent JoinAcross from deleting the whole thing.

Posted 8 days ago

In the SQL world, "keys" must. be in both sets. So, I am surprised by that left joining to an empty set did not give you an error.

For example, if you try:

JoinAcross[{<|a -> 1, b -> X|>}, {<|b -> 10, c -> 3|>}, 
 Key[a], "Left"]

you get:

{<|a -> 1, b -> X, c -> Missing["Unmatched"]|>}

which is what I would expect.

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