探究MySQL中varchar的定义长度

UTF8字符集下：

SQL>create table test(id int auto_increment,name varchar(10),primary key(id));
SQL>insert into test values(null,'1234567890');
Query OK, 1 row affected (0.00 sec)
SQL>insert into test values(null,'一二三四五六七八九十');
Query OK, 1 row affected (0.00 sec)
SQL>insert into test values(null,'abcdefghig');
Query OK, 1 row affected (0.01 sec)
SQL>insert into test values(null,12345678901);
ERROR 1406 (22001): Data too long for column 'name' at row 1
SQL>insert into test values(null,'一二三四五六七八九十1');
ERROR 1406 (22001): Data too long for column 'name' at row 1
SQL>insert into test values(null,'一二三四五六七八九十一');
ERROR 1406 (22001): Data too long for column 'name' at row 1
SQL>select id,name,length(name),char_length(name) from test;

+----+--------------------------------+--------------+-------------------+
 | id | name | length(name) | char_length(name) |
 +----+--------------------------------+--------------+-------------------+
 | 1 | 1234567890 | 10 | 10 |
 | 2 | 一二三四五六七八九十 | 30 | 10 |
 | 3 | abcdefghig | 10 | 10 |
 +----+--------------------------------+--------------+-------------------+
 3 rows in set (0.00 sec)

GBK字符集下：

SQL>create table test(id int auto_increment,name varchar(10),primary key(id));
SQL>insert into test values(null,'1234567890');
Query OK, 1 row affected (0.00 sec)
SQL>insert into test values(null,'一二三四五六七八九十');
Query OK, 1 row affected (0.00 sec)
SQL>insert into test values(null,'abcdefghig');
Query OK, 1 row affected (0.01 sec)
SQL>insert into test values(null,12345678901);
ERROR 1406 (22001): Data too long for column 'name' at row 1
SQL>insert into test values(null,'一二三四五六七八九十1');
ERROR 1406 (22001): Data too long for column 'name' at row 1
SQL>insert into test values(null,'一二三四五六七八九十一');
ERROR 1406 (22001): Data too long for column 'name' at row 1
SQL>select id,name,length(name),char_length(name) from test;

+----+----------------------+--------------+-------------------+
 | id | name | length(name) | char_length(name) |
 +----+----------------------+--------------+-------------------+
 | 1 | 1234567890 | 10 | 10 |
 | 2 | 一二三四五六七八九十 | 20 | 10 |
 | 3 | abcdefghig | 10 | 10 |
 +----+----------------------+--------------+-------------------+
 3 rows in set (0.00 sec)

由此可见，varchar定义的长度的单位是字符，哪怕是1个多字节字符也是1个字符，如中文和英文字母都被当作1个字符来对待。

那么varchar能够定义的最大长度是多少呢？这个和你当前所使用的字符集有关。抛开字符，其最大长度为65535字节(这是最大行大小，由所有列共享)，而放在不同的字符集下，能够定义的最大长度就会有所不同，如UTF8下是21845。据说MySQL5中varchar的长度也为字符，而MySQL4中的则为字节，未经证实，感兴趣的有环境可以自己测下。

顺便补充一下，char数据类型定义的长度也为字符，其最大长度为255。

SQL>create table test(id int auto_increment,name char(5),primary key(id));
Query OK, 0 rows affected (0.09 sec)
SQL>insert into test values(null,'123');
Query OK, 1 row affected (0.00 sec)
SQL>insert into test values(null,'12345');
Query OK, 1 row affected (0.00 sec)
SQL>insert into test values(null,'一二三');
Query OK, 1 row affected (0.00 sec)
SQL>insert into test values(null,'一二三四五');
Query OK, 1 row affected (0.00 sec)
SQL>insert into test values(null,123456);
ERROR 1406 (22001): Data too long for column 'name' at row 1
SQL>insert into test values(null,'一二三四五1');
ERROR 1406 (22001): Data too long for column 'name' at row 1
SQL>select id,name,length(name),char_length(name) from test;

+----+-----------------+--------------+-------------------+
 | id | name | length(name) | char_length(name) |
 +----+-----------------+--------------+-------------------+
 | 1 | 123 | 3 | 3 |
 | 2 | 12345 | 5 | 5 |
 | 3 | 一二三 | 9 | 3 |
 | 4 | 一二三四五 | 15 | 5 |
 +----+-----------------+--------------+-------------------+
 4 rows in set (0.00 sec)