mysql中的NULL的判断

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>