mysql> select * from t1 ;
+------+------+
| a    | b    |
+------+------+
|      | NULL |
+------+------+
1 row in set (0.00 sec)

mysql> select * from t1 where a is null ;
+------+------+
| a    | b    |
+------+------+
|      | NULL |
+------+------+
1 row in set (0.00 sec)