The NOT IN / NULL trap

NOT IN over a subquery is the most common way to write a statement that can never be true. It throws nothing, logs nothing, and matching zero rows is a perfectly ordinary outcome for a DELETE, so the failure has no symptom at all except work that quietly does not happen.

Measured here: one playlist-cleanup statement deleted 0 rows and reported no error for as long as it existed, because the column it filtered against was nullable and mostly NULL.

The mechanism, exactly

x NOT IN (a, b, c) is defined as NOT (x = a OR x = b OR x = c). Comparing anything to NULL is NULL, not false. So with a NULL anywhere in the list: