sql - 如果发生 NULL，Where 子句拒绝行

Question

一个理论问题...

当触发下面给出的一组查询时...

Create table Temp1(C1 varchar(2))
Create table Temp2(C1 varchar(2))
insert into Temp1 Values('A'),(NULL),('B')
insert into Temp2 Values('B'),(NULL),('C'),(NULL)

select *from Temp1 A,Temp2 B
where A.C1 <> B.C1

...给出...

我用过A.C1 <> B.C1在 Where条款。

<小时/> 但我预计...

为了获得预期结果作为输出，我需要使用 ISNULL(A.C1,'') <> ISNULL(B.C1,'')在 Where条款。

我的问题是为什么我需要使用ISNULL每次都按预期获得输出，如 NULL不等于任何字符串数据。

Answer 1

引自here很完美:

The correct way to understand NULL is that it is not a value. Not “this is a NULL value” but “this NULL is not a value.” Everything either is a value, or it isn’t. When something is a value, it is “1,” or “hello,” or “green,” or “$5.00″ etc — but when something isn’t a value, it just isn’t anything at all. SQL represents “this has no value” by the special non-value NULL. When someone says “the NULL value,” one should mentally disagree, because there’s no such thing. NULL is the complete, total absence of any value whatsoever.

Null 不等于任何内容，因此比较总是失败。

例如尝试这个查询:

select *
from Temp2 B
where B.C1 = null 

它不会返回任何行!

您必须使用的句柄 null is null 或 is not null:

select *
from Temp1 A, Temp2 B
where A.C1 <> B.C1 or (A.C1 is null and B.C1 is not null) or (B.C1 is null and A.C1 is not null)

返回与使用 ISNULL 查询返回的值完全相同的值。