一 前言
varchar(N) N代表什么意思，能存放多少个中文字符？属于老生常谈的问题了，今天又被一个开发同事问我关于这个问题，索性写一篇文章来具体介绍一下。
二 理论知识
先说明一下 MySQL 历来版本对 varchar 的定义:
4.0版本以下，varchar(50)，指的是50字节，如果存放UTF8汉字时，只能存16个（每个中文3字节）
5.0版本以上，varchar(50)，指的是50字符，无论存放的是数字、字母还是UTF8中文（每个中文3字节），都可以存放50个
存储限制
需要额外占用字节存放字符的长度:小于255为1个字节，大于255则要2个字节
编码限制
gbk :每个字符最多占用2个字节
utf8:每个字符最多占用3个字节
utf8mb4 每个字符最多占用4个字节,中文占3个字节，emoji表情符号 占用4个字节
长度限制
MySQL定义行的长度不能超过65535，该数值限制了列的数目，比如char(128) utf8字符集,最多有65535/(128*3)=170个汉字。
三 测试
环境 Server version: 5.6.26-74.0-log Percona Server
mysql> create table t1
-> (id int NOT NULL AUTO_INCREMENT primary key,
-> name varchar(10)
-> ) engine=innodb default charset=utf8mb4;
Query OK, 0 rows affected (0.01 sec)
mysql> create table t2
-> (id int NOT NULL AUTO_INCREMENT primary key,
-> name varchar(10)
-> ) engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.01 sec)
mysql> create table t3
-> (id int NOT NULL AUTO_INCREMENT primary key,
-> name varchar(10)
-> ) engine=innodb default charset=gbk;
Query OK, 0 rows affected (0.01 sec)
utf8mb4 字符集
mysql> insert into t1(name) values('abcdfeghi');
Query OK, 1 row affected (0.00 sec)
mysql> insert into t1(name) values('abcdfeghij');
Query OK, 1 row affected (0.00 sec)
mysql> insert into t1(name) values('abcdfeghijk');
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> insert into t1(name) values('一二三四五六七八九十');
Query OK, 1 row affected (0.00 sec)
mysql> insert into t1(name) values('一二三四五六七八九十一');
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> show warnings;
+---------+------+-------------------------------------------+
| Level | Code | Message |
+---------+------+-------------------------------------------+
| Warning | 1265 | Data truncated for column 'name' at row 1 |
+---------+------+-------------------------------------------+
1 row in set (0.00 sec)
mysql> insert into t1(name) values('0123456789');
Query OK, 1 row affected (0.00 sec)
mysql> insert into t1(name) values('01234567890');
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> select id,name,length(name),char_length(name) from t1;
+----+--------------------------------+--------------+-------------------+
| id | name | length(name) | char_length(name) |
+----+--------------------------------+--------------+-------------------+
| 1 | abcdfeghi | 9 | 9 |
| 2 | abcdfeghij | 10 | 10 |
| 3 | abcdfeghij | 10 | 10 |
| 4 | 一二三四五六七八九十 | 30 | 10 |
| 5 | 一二三四五六七八九十 | 30 | 10 |
| 6 | 0123456789 | 10 | 10 |
| 7 | 0123456789 | 10 | 10 |
+----+--------------------------------+--------------+-------------------+
7 rows in set (0.00 sec)
utf8 字符集
mysql> insert into t2(name) values('abcdfeghi');
Query OK, 1 row affected (0.00 sec)
mysql> insert into t2(name) values('abcdfeghij');
Query OK, 1 row affected (0.00 sec)
mysql> insert into t2(name) values('abcdfeghijk');
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> insert into t2(name) values('一二三四五六七八九十');
Query OK, 1 row affected (0.00 sec)
mysql> insert into t2(name) values('一二三四五六七八九十一');
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> insert into t2(name) values('0123456789');
Query OK, 1 row affected (0.00 sec)
mysql> insert into t2(name) values('01234567890');
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> select id,name,length(name),char_length(name) from t2;
+----+--------------------------------+--------------+-------------------+
| id | name | length(name) | char_length(name) |
+----+--------------------------------+--------------+-------------------+
| 1 | abcdfeghi | 9 | 9 |
| 2 | abcdfeghij | 10 | 10 |
| 3 | abcdfeghij | 10 | 10 |
| 4 | 一二三四五六七八九十 | 30 | 10 |
| 5 | 一二三四五六七八九十 | 30 | 10 |
| 6 | 0123456789 | 10 | 10 |
| 7 | 0123456789 | 10 | 10 |
+----+--------------------------------+--------------+-------------------+
7 rows in set (0.00 sec)
gbk 字符集
mysql> insert into t3(name) values('abcdfeghi');
Query OK, 1 row affected (0.00 sec)
mysql> insert into t3(name) values('abcdfeghij');
Query OK, 1 row affected (0.00 sec)
mysql> insert into t3(name) values('abcdfeghijk');
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> insert into t3(name) values('一二三四五六七八九十');
Query OK, 1 row affected (0.01 sec)
mysql> insert into t3(name) values('一二三四五六七八九十一');
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> insert into t3(name) values('0123456789');
Query OK, 1 row affected (0.00 sec)
mysql> insert into t3(name) values('01234567890');
Query OK, 1 row affected, 1 warning (0.00 sec)
mysql> select id,name,length(name),char_length(name) from t3;
+----+--------------------------------+--------------+-------------------+
| id | name | length(name) | char_length(name) |
+----+--------------------------------+--------------+-------------------+
| 1 | abcdfeghi | 9 | 9 |
| 2 | abcdfeghij | 10 | 10 |
| 3 | abcdfeghij | 10 | 10 |
| 4 | 一二三四五六七八九十 | 20 | 10 |
| 5 | 一二三四五六七八九十 | 20 | 10 |
| 6 | 一二三四五六七八九十 | 20 | 10 |
| 7 | 0123456789 | 10 | 10 |
| 8 | 0123456789 | 10 | 10 |
+----+--------------------------------+--------------+-------------------+
8 rows in set (0.00 sec)
从上面的测试可以看出,目前版本中 varchar(N) 定义的长度的单位是字符,length(str)表示str占用的字节数，char_length(str)表示str占用的字符数。
不论什么字符集，对于数字和英文字母都是只占用1个字符，也占用一个字节。而中文汉字因字符集不同而不同。