mysql也有这个NULL常量

mysql> insert into test values(NULL);
Query OK, 1 row affected (0.00 sec)

查询NULL值得时候要用is null或者is not null

下边例子还说明几个小问题：

1.null在排序中算最小值(记得oracle里相反)

2.change column 为not null会将原先的not null更改为0

change column 为not null的测试

mysql> select * from test order by a;
+------+
| a |
+------+
| NULL |
| 1 |
+------+
2 rows in set (0.00 sec)

mysql> alter table test change column a a int not null;
Query OK, 2 rows affected (0.06 sec)
Records: 2 Duplicates: 0 Warnings: 1

mysql> select * from test order by a;
+---+
| a |
+---+
| 0 |
| 1 |
+---+
2 rows in set (0.00 sec)

mysql> insert into test values(NULL);
ERROR 1048: Column 'a' cannot be null