A query using NOT IN is returning zero rows even though you expect some results. What's the likely cause?

If the subquery in NOT IN returns ANY NULL value, NOT IN returns empty result. Reason: SQL evaluates 'column NOT IN (1, 2, NULL)' as 'column != 1 AND column != 2 AND column != NULL'. The last condition (column != NULL) evaluates to NULL (UNKNOWN), which causes the entire WHERE condition to fail for every row. Fix: 1) Filter NULLs in the subquery: NOT IN (SELECT dept_id FROM employees WHERE dept_id