MySQL 计算字段长度函数LENGTH()与CHAR_LENGTH()

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本文详细解释了MySQL中LENGTH()和CHAR_LENGTH()函数的区别，LENGTH()用于计算字符串的字节长度，而CHAR_LENGTH()用于计算字符数量。通过实例展示了在不同字符集下，如UTF8，这两个函数如何返回不同的结果。

MySQL中有两个函数用来计算字段的长度：LENGTH()、CHAR_LENGTH()。

LENGTH()函数的定义为(命令为：? LENGTH)：

Name: ‘LENGTH’
Description:
Syntax: LENGTH(str)

Returns the length of the string str, measured in bytes. A multibyte character counts as multiple bytes.
This means that for a string containing five 2-byte characters, LENGTH() returns 10, whereas CHAR_LENGTH() returns 5.

Examples: mysql> SELECT LENGTH(‘text’);
-> 4

CHAR_LENGTH()函数的定义为(命令为 ? CHAR_LENGTH)：

Name: ‘CHAR_LENGTH’
Description:
Syntax: CHAR_LENGTH(str)

Returns the length of the string str, measured in characters. A multibyte character counts as a single character.
This means that for a string containing five 2-byte characters, LENGTH() returns 10, whereas CHAR_LENGTH() returns 5.

LENGTH()用来计算字节长度。而CHAR_LENGTH()用来计算字符长度。

用来计算单字节字符二者无区别：

mysql> SELECT LENGTH("CHINA"), CHAR_LENGTH("CHINA");
+-----------------+----------------------+
| LENGTH("CHINA") | CHAR_LENGTH("CHINA") |
+-----------------+----------------------+
|               5 |                    5 |
+-----------------+----------------------+

用来计算多字接字符时区别如下(假设使用的字符集为UTF8，汉字在UTF8中占3字节):

mysql> SELECT LENGTH("中国"), CHAR_LENGTH("中国");;
+------------------+-----------------------+
| LENGTH("中国")   | CHAR_LENGTH("中国")   |
+------------------+-----------------------+
|                6 |                     2 |
+------------------+-----------------------+