Why did all 10 rows vanish?

Question

Accepted Answer

NOT IN (v1, v2, NULL) expands to col != v1 AND col != v2 AND col != NULL. col != NULL is UNKNOWN for every possible col value because NULL is never equal or unequal to anything. WHERE discards rows that are not TRUE, so all rows disappear. The safe fix is WHERE id NOT IN (SELECT manager_id FROM employees WHERE manager_id IS NOT NULL), or use NOT EXISTS.