MySQL - length() 与 char_length()
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MySQL - length() vs char_length()
提问by Yada
What's the main difference between length()and char_length()?
length()和之间的主要区别是char_length()什么？
I believe it has something to do with binary and non-binary strings. Is there any practical reason to store strings as binary?
我相信它与二进制和非二进制字符串有关。将字符串存储为二进制有什么实际原因吗？
mysql> select length('MySQL'), char_length('MySQL');
+-----------------+----------------------+
| length('MySQL') | char_length('MySQL') |
+-----------------+----------------------+
| 5 | 5 |
+-----------------+----------------------+
1 row in set (0.01 sec)
回答by Andomar
LENGTH()returns the length of the string measured in bytes. CHAR_LENGTH()returns the length of the string measured in characters.
LENGTH()返回以字节为单位的字符串长度。 CHAR_LENGTH()返回以字符为单位的字符串长度。
This is especially relevant for Unicode, in which most characters are encoded in two bytes. Or UTF-8, where the number of bytes varies. For example:
这与 Unicode 尤其相关，其中大多数字符以两个字节进行编码。或 UTF-8，其中字节数不同。例如：
select length(_utf8 ''), char_length(_utf8 '')
--> 3, 1
As you can see the Euro sign occupies 3 bytes (it's encoded as 0xE282ACin UTF-8) even though it's only one character.
正如您所看到的，欧元符号占用 3 个字节（它0xE282AC以 UTF-8编码），即使它只有一个字符。
回答by Brent
varchar(10) will store 10 characters, which may be more than 10 bytes. In indexes, it will allocate the maximium length of the field - so if you are using UTF8-mb4, it will allocate 40 bytes for the 10 character field.
varchar(10) 将存储 10 个字符，可能超过 10 个字节。在索引中，它将分配字段的最大长度 - 因此，如果您使用 UTF8-mb4，它将为 10 个字符的字段分配 40 个字节。
