T-SQL 查询在声明整数与在查询中计算时给出不同的结果

问题描述

所以我一直在尝试在查询中生成一些 SQL 随机数，我注意到一些奇怪的事情.

So I've been experimenting with some SQL random number generation in a query, and I noticed something odd.

假设我运行以下查询:

declare @Random int = CAST(RAND(CHECKSUM(NEWID())) * 5 as INT) + 1;

select CHOOSE(@Random,'One','Two','Three','Four','Five')

SQL random number gen 有点笨重，但这里的基本思想很简单 - 在 1 到 5 之间选择一个随机数，然后将该数字作为文本显示到选择窗口.这按预期工作.

SQL random number gen is a little bulky, but the basic idea here is simple - pick a random number between 1 and 5, then display that number as text to the select window. This works as expected.

但是，如果我采用 SAME 查询，但将随机数公式粘贴到方法中而不是将其声明为整数，则所有内容都在一行中:

However, if I take the SAME query, but paste the random number formula into the method instead of declaring it as an integer, so it's all on one line:

select CHOOSE(CAST(RAND(CHECKSUM(NEWID())) * 5 as INT) +
    1,'One','Two','Three','Four','Five')

当我运行查询时，我仍然得到值一到五，但有时我也会得到一个 NULL.NULL 经常出现，大约五分之一.如果我将两个查询都放入 ssms 并并排运行几次，我经常会看到第二个查询的 null 值，但第一个查询永远不会为 NULL.

I still get values One to Five when I run the query, but sometimes I also get a NULL. The NULL comes up pretty often, about one in 5 times. If I put both queries into ssms and run them next to each other a few times, I often see a null value for the second query, but the first is never NULL.

那是为什么?这不是完全一样的计算吗?我不知道为什么这两个查询会给出不同的结果，但我觉得通过查找我可以学到一些关于 T-SQL 的有用信息.

So why is that? Aren't these exactly the same calculation? I have no idea why these two queries would give different results, but I feel like I can learn something useful about T-SQL by finding out.

有哪位高手给我指点一下吗?

Any experts want to enlighten me?

推荐答案

这是 SQL Server 中 choose 函数的一个非常微妙的问题(好吧，Microsoft 可能认为它是一个功能).该函数实际上是 case 表达式的简写.所以，你的表情:

This is a very subtle problem with the choose function in SQL Server (well, Microsoft probably considers it a feature). The function is really shorthand for a case expression. So, your expression:

select CHOOSE(CAST(RAND(CHECKSUM(NEWID())) * 5 as INT) +
1,'One','Two','Three','Four','Five')

翻译成:

select (case when CAST(RAND(CHECKSUM(NEWID())) * 5 as INT) + 1 = 1 then 'One'
             when CAST(RAND(CHECKSUM(NEWID())) * 5 as INT) + 1 = 2 then 'Two'
             when CAST(RAND(CHECKSUM(NEWID())) * 5 as INT) + 1 = 3 then 'Three'
             when CAST(RAND(CHECKSUM(NEWID())) * 5 as INT) + 1 = 4 then 'Four'
             when CAST(RAND(CHECKSUM(NEWID())) * 5 as INT) + 1 = 5 then 'Five'
        end)

这意味着 newid() 被多次调用.这通常不是您想要的行为.

That means that the newid() is being called multiple times. This is usually not the behavior that you want.

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