You could solve that by altogether dropping the Venn diagram metaphor when reasoning about joins. This is the number one problem I see with junior devs who have a hard time grokking SQL. If you think about a join as a cartesian product with a filter, where the type of join defines the type of filter, the reasoning is extremely easy.

Here's a good article about that: https://blog.jooq.org/2016/07/05/say-no-to-venn-diagrams-whe...