mysql> CREATE TABLE t1 ( a INT );
Query OK, 0 rows affected (0.04 sec)

mysql> INSERT INTO t1 VALUES (1), (2), (NULL), (3), (NULL) ;
Query OK, 5 rows affected (0.00 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM t1 WHERE ISNULL(a);
+------+
| a    |
+------+
| NULL | 
| NULL | 
+------+
2 rows in set (0.00 sec)