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  • I have indeed read your many comments. When you say, "a Venn diagram, when properly interpreted, can represent inner vs outer join" do you mean when properly interpreted by the observer or the Venn diagram itself? If the latter, please draw it :) Commented Apr 24, 2019 at 16:08
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    I'm not sure what you are trying to say. I am talking about the standard interpretation of a Venn diagram as sets of elements. (Because some uses of diagrams don't even manage that.) "Properly" for an application includes saying what the sets and/or elements are. See comment at the top of this page with 50 upvotes re a Venn diagram for inner vs outer joins. I'll edit some of my comments into this question. I don't want a Venn diagram in this post. Commented Apr 24, 2019 at 16:31
  • I must admit that, despite my quick phrasing in comments, because SQL involves bags & nulls and SQL culture doesn't have common terminology to name & distinguish between relevant notions, it is non-trivial even to explain clearly how elements of a Venn diagram are 1:1 with output "rows", let alone input "rows". Or what inner or outer joins do, let alone their difference. "value" may or may not include NULL, "row" may be a list of values vs a slot in a table value or variable & "=" may be SQL "=" vs equality. Commented Apr 25, 2019 at 17:44
  • Similar to our Cartesian-product-vs-relational-product discussion, I suspect it is the case that the Venn diagrams make a lot of sense to folk who already understand the differences between the join types! Commented Apr 29, 2019 at 7:51
  • In the case of 'relational Cartesian product', that is a standard & reasonable name for a certain thing that people do generally understand & which is reasonably described as similar to a Cartesian product. In the case of SQL Venn diagrams, they don't make sense, people just assume they do, whether they do or don't understand the operators/differences. Commented Apr 29, 2019 at 11:15