mysql 中int和varchar的长度

mysql 字段中int后面所跟数字有何意义？ varchar后的数字又有何意义？

mysql> create table t(a int(1));

Query OK, 0 rows affected (0.10 sec)

mysql> insert into t values(123);

Query OK, 1 row affected (0.02 sec)



mysql> insert into t values(12345678);

Query OK, 1 row affected (0.03 sec)



mysql> select * from t;

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

| a        |

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

|      123 |

| 12345678 |

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

可见，int(1)并不表示一个字节。
如果更长的数字会不会报错？

mysql> insert into t values(1234567812345678);

ERROR 1264 (22003): Out of range value for column 'a' at row 1

mysql> insert into t values(2147483648);

ERROR 1264 (22003): Out of range value for column 'a' at row 1

mysql> insert into t values(2147483647);

Query OK, 1 row affected (0.03 sec)

int 型长度最大值是2^31 -1 ，加上有符号数，应该是四个字节的长度。

mysql> alter table t add column b int;

Query OK, 5 rows affected (0.25 sec)

mysql> insert into t values(2147483647,2147483648);

ERROR 1264 (22003): Out of range value for column 'b' at row 1

mysql> insert into t values(2147483647,2147483647);

Query OK, 1 row affected (0.03 sec)



mysql> select * from t;

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

| a          | b          |

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

|        123 |       NULL |

|   12345678 |       NULL |

|      65536 |       NULL |

| 1073741824 |       NULL |

| 2147483647 |       NULL |

| 2147483647 | 2147483647 |

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

6 rows in set (0.00 sec)

可见，int后是否跟数字与最大值没有关系。

再看char 和varchar后的数字

mysql> alter table t add column c char(2);

Query OK, 6 rows affected (0.17 sec)

Records: 6  Duplicates: 0  Warnings: 0



mysql> alter table t add column d varchar(2);

Query OK, 6 rows affected (0.17 sec)

Records: 6  Duplicates: 0  Warnings: 0



mysql> desc t;

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

| Field | Type       | Null | Key | Default | Extra |

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

| a     | int(1)     | YES  |     | NULL    |       |

| b     | int(11)    | YES  |     | NULL    |       |

| c     | char(2)    | YES  |     | NULL    |       |

| d     | varchar(2) | YES  |     | NULL    |       |

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

4 rows in set (0.01 sec)



mysql> insert into t values(2147483647,2147483647,'abc','abc');

ERROR 1406 (22001): Data too long for column 'c' at row 1

mysql> insert into t values(2147483647,2147483647,'ab','abc');

ERROR 1406 (22001): Data too long for column 'd' at row 1

mysql> insert into t values(2147483647,2147483647,'ab','ab');

Query OK, 1 row affected (0.02 sec)

可见char和varchar后的数字是存储长度。
那么对于中文字，会不会有字节数大于字数而溢出的问题呢？

mysql> insert into t values(2147483647,2147483647,'ab','中化');

Query OK, 1 row affected (0.02 sec)



mysql> select * from t;

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

| a          | b          | c    | d    |

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

|        123 |       NULL | NULL | NULL |

|   12345678 |       NULL | NULL | NULL |

|      65536 |       NULL | NULL | NULL |

| 1073741824 |       NULL | NULL | NULL |

| 2147483647 |       NULL | NULL | NULL |

| 2147483647 | 2147483647 | NULL | NULL |

| 2147483647 | 2147483647 | ab   | ab   |

| 2147483647 | 2147483647 | ab   | 中化 |

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

8 rows in set (0.00 sec)

中文字也是包含在2个字内的。

mysql> insert into t values(2147483647,2147483647,'ab','中化a');

ERROR 1406 (22001): Data too long for column 'd' at row 1

mysql> insert into t values(2147483647,2147483647,'ab','烎镕');

Query OK, 1 row affected (0.05 sec)



mysql> select * from t;

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

| a          | b          | c    | d    |

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

|        123 |       NULL | NULL | NULL |

|   12345678 |       NULL | NULL | NULL |

|      65536 |       NULL | NULL | NULL |

| 1073741824 |       NULL | NULL | NULL |

| 2147483647 |       NULL | NULL | NULL |

| 2147483647 | 2147483647 | NULL | NULL |

| 2147483647 | 2147483647 | ab   | ab   |

| 2147483647 | 2147483647 | ab   | 中化 |

| 2147483647 | 2147483647 | ab   | 烎镕 |

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

9 rows in set (0.01 sec)

多一个字节都不行。
看一下字符编码。mysql字符编码由服务器，数据库，表，字段四级组成。
在windows下的mysql设置：

mysql> show variables like "character%";

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

| Variable_name            | Value                    |

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

| character_set_client     | gbk                      |

| character_set_connection | gbk                      |

| character_set_database   | utf8                     |

| character_set_filesystem | binary                   |

| character_set_results    | gbk                      |

| character_set_server     | utf8                     |

| character_set_system     | utf8                     |

| character_sets_dir       | D:\mysql\share\charsets\ |

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

8 rows in set (0.00 sec)

mysql> show create table t;

 t     | CREATE TABLE `t` (

 `a` int(1) DEFAULT NULL,

 `b` int(11) DEFAULT NULL,

 `c` char(2) DEFAULT NULL,

 `d` varchar(2) DEFAULT NULL

 ENGINE=InnoDB DEFAULT CHARSET=utf8

linux下centos设置：

mysql> show variables like "character%";

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

| Variable_name            | Value                      |

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

| character_set_client     | utf8                       |

| character_set_connection | utf8                       |

| character_set_database   | utf8                       |

| character_set_filesystem | binary                     |

| character_set_results    | utf8                       |

| character_set_server     | utf8                       |

| character_set_system     | utf8                       |

| character_sets_dir       | /opt/mysql/share/charsets/ |

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

8 rows in set (0.00 sec)

mysql> show full fields from t;

mysql> show create database test;

CREATE DATABASE `test` /*!40100 DEFAULT CHARACTER SET utf8 */

字符编码是utf8，
因此，对于utf8中文字符，varchar后面的数字，无论是中文还是英文，都表示相应的字数，不用担心截断。

http://abloz.com/2011/09/19/mysql-in-int-and-varchar-length.html