mysql> create table t1 (a int(4)); Query OK, 0 rows affected (0.14 sec) mysql> insert into t1 values(15652); Query OK, 1 row affected (0.06 sec) mysql> insert into t1 values(156522); Query OK, 1 row affected (0.05 sec) mysql> insert into t1 values(15652256145); ERROR 1264 (22003): Out of range value adjusted for column 'a' at row 1 mysql> insert into t1 values(1565222); Query OK, 1 row affected (0.06 sec) mysql> insert into t1 values(15652222); Query OK, 1 row affected (0.08 sec) mysql> insert into t1 values(156522222); Query OK, 1 row affected (0.05 sec) mysql> insert into t1 values(1565222222); Query OK, 1 row affected (0.03 sec) mysql> insert into t1 values(1565222222)2; ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '2' at line 1 mysql> 为什么int(4)类型的字段可以插入9位的数字呢？

int(n)中不论n指定为多少，范围都一样-2~31~2^31-1，都能插入9位数字。n只是用来控制显示而已，如果为int(4)，插入一个1，显示时也会用空格补齐到4个字节

请问所有mysql中的数据类型中的(n)都是表示显示位数吗？

并非如此，NUMERIC（4）就限定了取值范围，超过4位就不允许写入了。因此如果你想限定数字的取值范围，最好是用NUMERIC来定义。