mysql查询在条件比较char与int 0时出现的一些现象 [英] some phenomenon of mysql query in where condition compare char filed with int 0

问题描述

有一张桌子，

root@localhost:[test]05:35:05>desc t;
+-----------+----------+------+-----+---------+----------------+
| Field     | Type     | Null | Key | Default | Extra          |
+-----------+----------+------+-----+---------+----------------+
| id        | int(11)  | NO   | PRI | NULL    | auto_increment |
| studio_id | char(32) | YES  |     | NULL    |                |
+-----------+----------+------+-----+---------+----------------+

具有两行:

root@localhost:[test]05:35:29>select * from t;
+----+----------------------------------+
| id | studio_id                        |
+----+----------------------------------+
|  1 | foo1                             |
|  2 | 299a0be4a5a79e6a59fdd251b19d78bb |
+----+----------------------------------+

发现了一些奇怪的查询现象，例如

found some strange query phenomenon, e.g.

# I can understand this
root@localhost:[test]05:37:00>select * from t where studio_id = '0';
Empty set (0.00 sec)
# I also understand this
root@localhost:[test]05:41:45>select * from t where studio_id = 1;
Empty set, 2 warnings (0.00 sec)


# but I can't understand this
root@localhost:[test]05:36:21>select * from t where studio_id = 0;
+----+-----------+
| id | studio_id |
+----+-----------+
|  1 | foo1      |
+----+-----------+

为什么可以返回记录，为什么只返回foo1，299a0be4a5a79e6a59fdd251b19d78bb呢?

why could have record returned, and why only foo1 returned , what about 299a0be4a5a79e6a59fdd251b19d78bb?

root@localhost:[test]05:38:20>select * from t where studio_id <> 0;
+----+----------------------------------+
| id | studio_id                        |
+----+----------------------------------+
|  2 | 299a0be4a5a79e6a59fdd251b19d78bb |
+----+----------------------------------+

推荐答案

原因是mysql如何如

The cause is how mysql silently converts text to numbers to evaluate the number=text expression as described in mysql's documentation on type conversion in expression evaluation.

Number =文本比较表达式是通过将两个操作数都转换为浮点数来求值的.

Number=text comparison expressions are evaluated by converting both operands to floating point numbers.

通过从左到右评估字符将文本转换为数字.只要可以将字符评估为有效数字(符号，数字，小数点等)，mysql就会将其视为数字.当mysql遇到无法在数字中使用的字符(例如字母)或将导致无效数字(例如第二个符号)时，转换将停止.

Text is converted to a number by evaluating the characters from left to right. As long as the characters can be evaluated as a valid number (sign, numbers, decimal point, etc), mysql considers it as a number. The conversion stops in the moment mysql encounters a character that cannot be used in a number (such as a letter), or would result in an ivalid number (such as a 2nd sign).

文本'foo1'被转换为0，因为其最左边的字符是字母.

Text 'foo1' is converted to 0 because its leftmost character is a letter.

文本'299a0be4a5a79e6a59fdd251b19d78bb'被转换为299，因为它以字符299开头，然后出现字母a，该字母不能解释为数字.

Text '299a0be4a5a79e6a59fdd251b19d78bb' is converted to 299 because it starts with the characters 299 and then comes the letter a that cannot be interpreted as a number.

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