INTERSECT when both tables contain NULL

Question

Accepted Answer

Set operations treat two NULLs as equal for the purpose of matching rows - unlike the = operator, where NULL = NULL evaluates to UNKNOWN. Because NULL appears in both t1 and t2, INTERSECT considers it a match and includes it in the output. This is one of the few places in SQL where NULL behaves like a regular value for comparison purposes, and it catches many developers off guard.