The WHERE clause trap every beginner hits

Question

Accepted Answer

dept_id = NULL produces UNKNOWN for every row, including rows where dept_id is itself NULL. UNKNOWN is never TRUE, so the WHERE clause never passes any row. To find rows with no department, write `WHERE dept_id IS NULL`. This is the single most common NULL mistake in SQL - always reach for IS NULL, never = NULL.

id	name	dept_id
1	Alice	10
2	Bob	NULL
3	Carol	NULL