MySQL MOD()已损坏:这是最好的选择吗? [英] MySQL MOD() is broken: Is this the best alternative?

问题描述

至少在MySQL v5.1.73(CentOS 6.6)中，MOD()函数返回假结果...除非有人可以解释这实际上是正确的.

At least in MySQL v5.1.73 (CentOS 6.6), MOD() function returns a bogus result... unless someone can explain how this is actually correct.

mysql> select MOD(-385.4784399 ,1440);
+-------------------------+
| MOD(-385.4784399 ,1440) |
+-------------------------+
|            -385.4784399 |
+-------------------------+
1 row in set (0.01 sec)

这是最好的选择吗?

mysql> select -385.478439885319 - 1440 * FLOOR(-385.478439885319/1440);
+----------------------------------------------------------+
| -385.478439885319 - 1440 * FLOOR(-385.478439885319/1440) |
+----------------------------------------------------------+
|                                        1054.521560114681 |
+----------------------------------------------------------+
1 row in set (0.00 sec)

我认为这也可以，但是要做一些简单的事情都需要很长的路要走.

I think this will work as well, but it's all going the long way around to do something simple.

mysql> select MOD((MOD(-385.478439885319, 1440) + 1440), 1440);
+--------------------------------------------------+
| MOD((MOD(-385.478439885319, 1440) + 1440), 1440) |
+--------------------------------------------------+
|                                1054.521560114681 |
+--------------------------------------------------+
1 row in set (0.00 sec)

推荐答案

MySQL并没有给您带来虚假的结果，它只是使用了与预期不同的模数实现.不幸的是，术语模数"似乎定义不明确，并且其实现因语言而异.从关于Modulo的维基百科中我可以看出，MySQL的实现使用了截断除法:

MySQL isn't giving you a bogus result, it's simply using a different implementation of modulus than you are expecting. Unfortunately the term modulus seems to have been defined somewhat ambiguously, and it's implementation varies from language to language. From what I can tell in the Wikipedia on Modulo, MySQL's implementation is using truncated division:

r = a - n * trunc(a / n)

您希望该实施使用地板除法:

r = a - n * floor(a / n)

由于这是您实施第一个解决方法的方式，因此我想这可能是Mod运算符的最佳替代方法.

Since this how you implemented your first workaround, I'd say it's probably the best alternative to the Mod operator.

从我所见(这是一个非常快速的不科学的分析！)看来，似乎更命令式的编程语言实现了截断除法，而更多的函数数学语言似乎使用了部门.

From what I've seen (and this is a very quick unscientific analysis!), it seems like more imperative programming languages implement truncated division and more functional mathematical languages seem to use floored division.

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