mysql> delete from `zhangchao` where x='NULL';

Query OK, 0 rows affected (0.00 sec)

mysql> delete from `zhangchao` where x=NULL;

Query OK, 0 rows affected (0.00 sec)

mysql> select * from zhangchao;

+------+-------+

| x    | y     |

+------+-------+

| xxx  | yyy   |

| NULL | NULL  |

| xxx  | james |

| xxx  | lucy  |

| xxx  | alex  |

+------+-------+

5 rows in set (0.00 sec)

mysql> delete from `zhangchao` where x  is null;

Query OK, 1 row affected (0.00 sec)

mysql> select * from zhangchao;

+------+-------+

| x    | y     |

+------+-------+

| xxx  | yyy   |

| xxx  | james |

| xxx  | lucy  |

| xxx  | alex  |

+------+-------+

4 rows in set (0.00 sec)

mysql>