mysql 中int和varchar的长度

周海汉
abloz.com
2011.9.19
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。