Stop Using Venn Diagrams for SQL Joins: The "Row Filter" Mental Model Stop Using Venn Diagrams for SQL Joins: The "Row Filter" Mental Model The most common way SQL is taught is also the most misleading. It's time to delete the circles from your brain and start thinking in rows. If you learned SQL in a bootcamp or a university, you were almost certainly shown a Venn Diagram. You know the one: two intersecting circles labeled "Table A" and "Table B." The intersection is labeled "Inner Join." Here is the problem: SQL is not Set Theory. When you rely on Venn Diagrams, you start to think of tables as sets of objects. But when you write a query and half your customer data vanishes, the Venn Diagram doesn't explain why. It doesn't explain duplication. It doesn't explain NULL values. The Truth: ...