IS NULL / IS NOT NULL演算子は、指定されたカラムがNULL値を持つかどうかを確認します。
IS NULL / IS NOT NULL
mysql> SELECT * FROM users;
+----+-------+------+
| id | name | age |
+----+-------+------+
| 1 | user1 | 10 |
| 2 | user2 | NULL |
| 3 | user3 | 30 |
| 4 | user4 | NULL |
| 5 | user5 | 50 |
+----+-------+------+
5 rows in set (0.00 sec)
【年齢がNULLなもの】
mysql> SELECT * FROM users where age IS NULL;
+----+-------+------+
| id | name | age |
+----+-------+------+
| 2 | user2 | NULL |
| 4 | user4 | NULL |
+----+-------+------+
2 rows in set (0.00 sec)
【年齢がNULLではないもの】
mysql> SELECT * FROM users where age IS NOT NULL;
+----+-------+------+
| id | name | age |
+----+-------+------+
| 1 | user1 | 10 |
| 3 | user3 | 30 |
| 5 | user5 | 50 |
+----+-------+------+
3 rows in set (0.00 sec)