NULL and Three-Valued Logic

NULL Means ‘No Value’

NULL means there is no value, and an ordinary comparison cannot judge it. city = NULL is never true — you use city IS NULL. AND and OR evaluate over three values (true, false, unknown), and a NULL inside a NOT IN list can leave the whole result empty.

GOAL

Explain why = NULL fails and use IS NULL correctly.

SELECT * FROM customers WHERE city = NULL returns how many rows?

CHECK IT

SELECT *
FROM customers
WHERE city = NULL

Matches the condition (kept)abcDoesn't match (dropped)

customers0 of 6 rows kept

id	name	city	referred_by
1	Alice	Tokyo	NULL
2	Bob	Osaka	1
3	Carol	Tokyo	2
4	Dave	Osaka	1
5	Erin	Nagoya	NULL
6	Frank	NULL	3

NULL means ‘no value’, and ordinary comparisons cannot judge it.