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I would also like to know why you are getting downvoted. Even wikipedia uses a Venn diagram to explain JOIN https://en.wikipedia.org/wiki/Join_(SQL) https://en

• radiospiel 3y ago
  Venn diagrams are a terrible way to describe join types (and, tbh, I don’t understand why Wikipedia has these) because it makes it look like applying an 1:1 relationship. In a M:N relationship „artefacts“ (for lack of better words) of both tables would appear multiple times, and the venn diagram obscures this fact
  • ericHosick 3y ago
    > because it makes it look like applying an 1:1 relationship. People can form different mental models of the same abstraction so I see what you are saying I've never seen it that way because "Venn diagrams do not generally contain information on the relative or absolute sizes (cardinality) of sets." (see https://en.wikipedia.org/wiki/Venn_diagram https://en.wikipedia.org/wiki/Venn_diagram).