环境：CentOS5.x，MySQL5.1.x
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)
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