为什么MySQL VARCHAR允许超过最大长度？

Question

我创建了一个带有UTF-8的表VARCHAR(5000),并用数据填充它.但看起来这个字段允许的数据多于指示的数据:

mysql> DESCRIBE test;
+---------+------------------+------+-----+---------+----------------+
| Field   | Type             | Null | Key | Default | Extra          |
+---------+------------------+------+-----+---------+----------------+
| id      | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| comment | varchar(5000)    | YES  |     | NULL    |                |
+---------+------------------+------+-----+---------+----------------+
2 rows in set (0.01 sec)

mysql> SELECT MAX(LENGTH(comment)) FROM test;
+----------------------+
| MAX(LENGTH(comment)) |
+----------------------+
|                 5001 |
+----------------------+
1 row in set (0.01 sec)

这是为什么？

Answer 1

好吧,问题是LENGTH()以字节为单位返回长度,而不是字符.因为字符串是UTF-8,我需要使用CHAR_LENGTH()代替:

mysql> DESCRIBE test;
+---------+------------------+------+-----+---------+----------------+
| Field   | Type             | Null | Key | Default | Extra          |
+---------+------------------+------+-----+---------+----------------+
| id      | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| comment | varchar(5000)    | YES  |     | NULL    |                |
+---------+------------------+------+-----+---------+----------------+
2 rows in set (0.01 sec)

mysql> SELECT MAX(LENGTH(comment)) FROM test;
+----------------------+
| MAX(LENGTH(comment)) |
+----------------------+
|                 5001 |
+----------------------+
1 row in set (0.01 sec)

mysql> SELECT MAX(CHAR_LENGTH(comment)) FROM test;
+----------------------+
| MAX(LENGTH(comment)) |
+----------------------+
|                 5000 |
+----------------------+
1 row in set (0.01 sec)

长度为5001,因为字符串只包含一个双字节字符!