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Inner join vs. outer join

From: Elias Mårtenson
Subject: Inner join vs. outer join
Date: Tue, 14 Apr 2020 12:06:57 +0800

I was reading the spec on outer join to see if I could finally fully understand how it works.

For reference, the spec refers to the syntax as follows:

    X a.b Y

The function a is referenced twice in the description for this function, both times in the form a/ (i.e. with the reduce operator applied).

I then noted that the J version of this function does not apply the reduce operator. I.e. in J, you have to do:

    X a/.b Y

Now, am I correct in my assumption that with this change, there would be no need for the ∘ (null function)? I.e. if APL behaved like J, would the following achieve an outer join?

    X ⊣.× Y

I'm still not sure I fully understand the join operator to the point where I can answer this question.


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