奇怪的CAST行为与INT和单个连字符（ - ） [英] Strange CAST behaviour with INT and a single hyphen ( - )

问题描述

我刚刚回答了这个问题： SQL中的主要键值

I just answered this question: Concate Primary Keys in SQL

我遇到了一个奇怪的行为：

There I encountered a strange behaviour:

SELECT 5 + '-' +  8 

返回13

SELECT CAST('-' AS INT) 

返回0，说明上述...

returns 0, which explains the above...

但是：为什么单个连字符隐含地隐藏？

But: Why is a single hyphen casted to zero implictly?

Btw：与单个+ - ）blank string ...

Btw: It's the same with a single + or a (multi-)blank string...

这是相关的 CAST连字符（ - ）到十进制，但指向这样的事实，即转换为小数不会带来这些结果...

This is related CAST hyphen (-) to Decimal but points to the fact, that a cast to decimal would not bring up these results...

推荐答案

我写这个答案，因为它太长/复杂的评论 - 但它是基于我的意见。
另外请注意 - 我没有一个官方的源，所以没有确认我的逻辑是什么实现。 （但我认为这是有道理的：））

I'm writing this as an answer because it's too long/complex for a comment - but it's based off my comments. Also note - I DO NOT HAVE AN OFFICIAL SOURCE, so no confirmation that my logic is "what's implemented". (But it makes sense I think :))

但假设你正在写一个需要执行的转换函数。

But suppose you're writing a conversion function, that needs to perform.

所以你有一个字符串来验证 - 例如 CAST（' - 50'AS INT）;

然后将每个字符它自己的：

So you have a string to validate - for example CAST('-50' AS INT);
Then take each character on its own:

`-` is a valid part of the conversion, so move to next character.  
`5` is a valid part of the conversion, so move to next character.  
`0` is a valid part of the conversion, so move to next character.  
Done.

所以假定字符串 CAST（' - 'AS INT） code>：

So supposed the string CAST('-' AS INT);:

`-` is a valid part of the conversion, so move to next character.  
Done.

现在 - 您可以执行额外的检查以使无效，因为 - 不允许独立，但这将需要额外的代码。

Now - you could do an additional check to invalidate because - is not allowed standalone, but that would require additional code.

与 + 和空格类似。
以及货币和金额或期间和金额相同：

Similar with + and spaces. As well as the same with currency and MONEY or period and money:

SELECT CAST('$' AS MONEY)
SELECT CAST('.' AS MONEY)

两者都是有效字符实际只与实际数字有关。但他们解析通过 - 表明这是所有故意，​​和转换的速度似乎是明智的。

Both are valid characters - but in actuality only in connection with actual numbers as well. But they parse through - indicating that it's all intentional, and speed for conversion seems sensible.

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