与 NULL 合并

2021-09-10 00:00:00 sql tsql sql-server coalesce

我在一个视图中发现了这个 SQL 片段，但我对它的用途感到相当困惑(为简洁起见，实际 SQL 已缩短):

I found this snippet of SQL in a view and I am rather puzzled by it's purpose (actual SQL shortened for brevity):

SELECT
    COALESCE(b.Foo, NULL) AS Foo

FROM a
LEFT JOIN b ON b.aId=a.Id

我想不出与 null 合并的目的的单一原因，而不仅仅是这样做:

I cannot think of a single reason of the purpose of coalescing with null instead of just doing this:

SELECT
    b.Foo AS Foo

FROM a
LEFT JOIN b ON b.aId=a.Id

或者至少不要显式包含 NULL:

Or at the very least don't include the NULL explicitly:

SELECT
    COALESCE(b.Foo) AS Foo

FROM a
LEFT JOIN b ON b.aId=a.Id

我不知道这是谁创作的(所以我不能问)，它是什么时候创作的，或者它是为哪个特定的 MS SQL Server 版本编写的(当然是 2008 年之前的版本).

I don't know who authored this (so I cannot ask), when it was authored, or for what specific MS SQL Server version it was written for (pre-2008 for sure though).

是否有任何正当理由与 NULL 合并而不是直接选择列? 我忍不住笑着把它写成菜鸟错误，但这让我想知道是否有一些我不知道的边缘案例".

Is there any valid reason to coalesce with NULL instead of just selecting the column directly? I can't help but laugh and write it off as a rookie mistake but it makes me wonder if there is some "fringe case" that I don't know about.

推荐答案

你说得对 - 没有理由使用:

You are right - there is no reason to use:

SELECT COALESCE(b.Foo, NULL)

...因为如果 b.foo 为 NULL，你不妨使用:

...because if b.foo is NULL, you might as well just use:

SELECT b.foo

...假设您想知道该值是否为空.

...assuming that you want to know if the value is null.

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