mysql - 为什么是 MySQL ... 为什么？

Question

那么，对于 mysql，为什么彼此不相等的事物会彼此相等？例如，为什么...

mysql> SELECT '3' = 3;
+---------+
| '3' = 3 |
+---------+
|       1 |
+---------+

只是，为什么？

更重要的是......

mysql> SELECT 0 = '';
+--------+
| 0 = '' |
+--------+
|      1 |
+--------+

但是，为什么？

还有……

mysql> SELECT '3x' into @foo;
Query OK, 1 row affected (0.00 sec)

mysql> SELECT @foo, CAST(@foo as signed), @foo = CAST(@foo as signed);
+------+----------------------+-----------------------------+
| @foo | CAST(@foo as signed) | @foo = CAST(@foo as signed) |
+------+----------------------+-----------------------------+
| 3x   |                    3 |                           1 |
+------+----------------------+-----------------------------+

亲爱的上帝，为什么？

但更糟糕的是......

mysql> SELECT '3x', CAST('3z' as signed), '3x' = CAST('3z' as signed);
+----+----------------------+-----------------------------+
| 3x | CAST('3z' as signed) | '3x' = CAST('3z' as signed) |
+----+----------------------+-----------------------------+
| 3x |                    3 |                           1 |
+----+----------------------+-----------------------------+

为什么，哦，为什么？为什么它让我哭得这么......？

Answer 1

文档里都有:

http://dev.mysql.com/doc/refman/5.1/en/type-conversion.html

尤其是行

In all other cases, the arguments are compared as floating-point (real) numbers.

所以你所有的比较都是浮点比较，因此非常有意义。

人们可以争论自动类型转换是否有意义(是否可以将 '1' 与 1 进行比较？)...