CHAR_LENGTH() ：Return number of characters（字符） in argument（参数）
LENGTH() ： Return the length of a string（字符串） in bytes（字节）
https://dev.mysql.com/doc/refman/5.5/en/string-functions
官方说明，CHAR_LENGTH() 返回变量中字符的个数，LENGTH() 返回一个字符串占多少字节。
个人英语比较马虎，所以不知道翻译对不对，凑合看。
结论：
gbk-gbk 一个中文字符也是一个字符，一个汉子占两个字节，所以是 5 7 gbk - utf8 不能直接转换，需通过unicode转码，转码后就是 7 7 gbk- latin1 可直接转换，转换为单字节字符，转码后是 7 7 utf8(一个中文三个字节)---gbk: 直接转换,尾部奇数字节截断（中文个数*3/2+英文字符个数） 9 9 utf8(一个中文三个字节)---utf8: 不用转换，就是 5 9 utf8--------latin1: 直接转换，单字节字符 就是 9 9
select char_length('世界sad'),length('世界sad');的结果跟两个因素有关：
客户端（linux默认utf8,windows默认GB2312，可通过securecrt 模拟），mysql字符编码设置（主要是client，connection和result,set names xxx）
测试1：
mysql version:5.6.17 windows 7 平台
mysql> set names utf8;
Query OK, 0 rows affected (0.00 sec)
mysql> select char_length('世界sad'),length('世界sad');
+------------------------+-------------------+
| char_length('世界sad') | length('世界sad') |
+------------------------+-------------------+
| 7 | 7 |
+------------------------+-------------------+1 row in set (0.00 sec)
mysql> show variables like 'char%';
+--------------------------+-----------------------------------------------+
| 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 | latin1 |
| character_set_system | utf8 |
| character_sets_dir | D:\wamp\bin\mysql\mysql5.6.17\share\charsets\ |
+--------------------------+-----------------------------------------------+8 rows in set (0.00 sec)
mysql> set names gbk;
Query OK, 0 rows affected (0.00 sec)
mysql> select char_length('世界sad'),length('世界sad');
+------------------------+-------------------+
| char_length('世界sad') | length('世界sad') |
+------------------------+-------------------+
| 5 | 7 |
+------------------------+-------------------+1 row in set (0.00 sec)
mysql> set names latin1;
Query OK, 0 rows affected (0.00 sec)
mysql> select char_length('世界sad'),length('世界sad');
+------------------------+-------------------+
| char_length('世界sad') | length('世界sad') |
+------------------------+-------------------+
| 7 | 7 |
+------------------------+-------------------+1 row in set (0.00 sec)
测试2：
mysql 5.5 tlinux 1.2/centos 6.x
mysql> set names utf8;Query OK, 0 rows affected (0.00 sec)
mysql> select char_length('世界sad'),length('世界sad');+--------------------------+---------------------+| char_length('世界sad') | length('世界sad') |
+--------------------------+---------------------+| 5 | 9 |
+--------------------------+---------------------+1 row in set (0.00 sec)
mysql> set names gbk;Query OK, 0 rows affected (0.00 sec)
mysql> select char_length('世界sad'),length('世界sad');+--------------------------+---------------------+| char_length('世界sad') | length('世界sad') |
+--------------------------+---------------------+| 6 | 9 |
+--------------------------+---------------------+1 row in set (0.00 sec)
mysql> set names latin1;Query OK, 0 rows affected (0.00 sec)
mysql> select char_length('世界sad'),length('世界sad');+--------------------------+---------------------+| char_length('世界sad') | length('世界sad') |
+--------------------------+---------------------+| 9 | 9 |
+--------------------------+---------------------+1 row in set (0.00 sec)
mysql>
测试3： mysql 5.6 tlinux1.2
elcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 1Server version: 5.6.25 Source distribution
Copyright (c) 2000, 2015, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> select char_length('世界sad'),length('世界sad');
+--------------------------+---------------------+
| char_length('世界sad') | length('世界sad') |
+--------------------------+---------------------+
| 5 | 9 |
+--------------------------+---------------------+1 row in set (0.00 sec)
mysql> show variables like 'char%';
+--------------------------+-----------------------------------------+
| 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 | /usr/local/mysql-5.6.25/share/charsets/ |
+--------------------------+-----------------------------------------+8 rows in set (0.00 sec)
mysql> set names gbk;
Query OK, 0 rows affected (0.00 sec)
mysql> show variables like 'char%';
+--------------------------+-----------------------------------------+
| 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 | /usr/local/mysql-5.6.25/share/charsets/ |
+--------------------------+-----------------------------------------+8 rows in set (0.00 sec)
mysql> select char_length('世界sad'),length('世界sad');
+--------------------------+---------------------+
| char_length('世界sad') | length('世界sad') |
+--------------------------+---------------------+
| 6 | 9 |
+--------------------------+---------------------+1 row in set (0.02 sec)
mysql> set names latin1;
Query OK, 0 rows affected (0.00 sec)
mysql> show variables like 'char%';
+--------------------------+-----------------------------------------+
| Variable_name | Value |
+--------------------------+-----------------------------------------+
| character_set_client | latin1 |
| character_set_connection | latin1 |
| character_set_database | utf8 |
| character_set_filesystem | binary |
| character_set_results | latin1 |
| character_set_server | utf8 |
| character_set_system | utf8 |
| character_sets_dir | /usr/local/mysql-5.6.25/share/charsets/ |
+--------------------------+-----------------------------------------+8 rows in set (0.00 sec)
mysql> select char_length('世界sad'),length('世界sad');
+--------------------------+---------------------+
| char_length('世界sad') | length('世界sad') |
+--------------------------+---------------------+
| 9 | 9 |
+--------------------------+---------------------+1 row in set (0.00 sec)
mysql>
测试4：
mariadb 5.5 tlinux 2.0/centos 7
[root@tlinux2_ops01 ~]# rpm -qa |grep mariadbmariadb-devel-5.5.41-2.el7_0.x86_64
mariadb-libs-5.5.41-2.el7_0.x86_64
mariadb-server-5.5.41-2.el7_0.x86_64
mariadb-embedded-devel-5.5.41-2.el7_0.x86_64
mariadb-embedded-5.5.41-2.el7_0.x86_64
mariadb-test-5.5.41-2.el7_0.x86_64
mariadb-5.5.41-2.el7_0.x86_64
mariadb-bench-5.5.41-2.el7_0.x86_64
[root@tlinux2_ops01 ~]# mysqlWelcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 58Server version: 5.5.41-MariaDB-log MariaDB Server
Copyright (c) 2000, 2014, Oracle, MariaDB Corporation Ab and others.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
MariaDB [(none)]> show variables like 'char%';show variables like 'char%';
+--------------------------+----------------------------+
| Variable_name | Value |
+--------------------------+----------------------------+
| character_set_client | latin1 |
| character_set_connection | latin1 |
| character_set_database | latin1 |
| character_set_filesystem | binary |
| character_set_results | latin1 |
| character_set_server | latin1 |
| character_set_system | utf8 |
| character_sets_dir | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+8 rows in set (0.00 sec)
+--------------------------+----------------------------+
| Variable_name | Value |
+--------------------------+----------------------------+
| character_set_client | latin1 |
| character_set_connection | latin1 |
| character_set_database | latin1 |
| character_set_filesystem | binary |
| character_set_results | latin1 |
| character_set_server | latin1 |
| character_set_system | utf8 |
| character_sets_dir | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+8 rows in set (0.00 sec)
MariaDB [(none)]> select char_length('世界sad'),length('世界sad');
+--------------------------+---------------------+
| char_length('世界sad') | length('世界sad') |
+--------------------------+---------------------+
| 9 | 9 |
+--------------------------+---------------------+1 row in set (0.00 sec)
MariaDB [(none)]> set names gbk;
Query OK, 0 rows affected (0.00 sec)
MariaDB [(none)]> select char_length('世界sad'),length('世界sad');
+--------------------------+---------------------+
| char_length('世界sad') | length('世界sad') |
+--------------------------+---------------------+
| 6 | 9 |
+--------------------------+---------------------+1 row in set (0.00 sec)
MariaDB [(none)]> set names utf8;
Query OK, 0 rows affected (0.00 sec)
MariaDB [(none)]> select char_length('世界sad'),length('世界sad');
+--------------------------+---------------------+
| char_length('世界sad') | length('世界sad') |
+--------------------------+---------------------+
| 5 | 9 |
+--------------------------+---------------------+1 row in set (0.00 sec)
MariaDB [(none)]>
测试5：
windows系统（终端设置为utf8） mysql 5.6
chcp 65001
