有没有办法简化 2 个值的 NULL 比较 [英] Is there a way to simplify a NULL compare of 2 values
问题描述
这是我的简单陈述
SELECT ...
FROM tab1 AS i FULL OUTER JOIN tab2 AS d ON i.[Id]=d.[Id]
WHERE d.[Data]<>i.[Data] OR
(d.[Data] IS NULL AND i.[Data] IS NOT NULL) OR
(d.[Data] IS NOT NULL AND i.[Data] IS NULL)
我想获取所有的条目
- i.[Data] 不同于 d.[Data]
- 表 i 或 d 中至少有一个值为 NOT NULL
所以我不想看到记录 was 和 i 和 d 包含相同的数据或者都是 NULL.
So I don't want to see records were and i and d contain the same data or are both NULL.
我的陈述看起来又长又复杂.有没有更简单的方法?
My statement look so long and complicated. Is there an easier way?
使用 ISNULL(d.[Data],'')<>ISNULL(i.[Data],'')
适用于文本,但不适用于 DATE
或 TIME(0)
列.
Using ISNULL(d.[Data],'')<>ISNULL(i.[Data],'')
works for text, but not for DATE
or TIME(0)
columns.
我的语句适用于所有类型.
My statement works for every type.
推荐答案
是的,你可以,而且你也可以让优化器识别它.
WHERE NOT EXISTS (
SELECT d.[Data]
INTERSECT
SELECT i.[Data])
这是因为 INTERSECT
处理空值的语义.这意味着在由值 B 和值 B 组成的子查询中是否有没有行",只有当它们是不同的值或者一个为空而另一个不是时才会满足.如果两者都是空值,则会有一行为空值.
This works because of the semantics of INTERSECT
which deal with nulls. What this says is "are there no rows in the subquery made up of value B and value B", this will only be satisfied if they are different values or one is null and the other not. If both are nulls, there will be a row with a null.
如果您检查 XML 查询计划(不是 SSMS 中的图形查询计划),您将看到它一直编译到 d.[Data] <>i.[Data]
,但它使用的运算符将具有 CompareOp="IS"
而不是 EQ
.
If you check the XML query plan (not the graphical one in SSMS), you will see that it compiles all the way down to d.[Data] <> i.[Data]
, but the operator it uses will have CompareOp="IS"
and not EQ
.
在此处查看完整计划.
该计划的相关部分是:
<Predicate>
<ScalarOperator ScalarString="@t1.[i] as [t1].[i] = @t2.[i] as [t2].[i]">
<Compare CompareOp="IS">
<ScalarOperator>
<Identifier>
<ColumnReference Table="@t1" Alias="[t1]" Column="i" />
</Identifier>
</ScalarOperator>
<ScalarOperator>
<Identifier>
<ColumnReference Table="@t2" Alias="[t2]" Column="i" />
</Identifier>
</ScalarOperator>
</Compare>
</ScalarOperator>
</Predicate>
我发现优化器以这种方式运行得非常好,而不是执行 EXISTS/EXCEPT
.
I find the optimizer works very well this way round, rather than doing EXISTS / EXCEPT
.
我敦促您投票支持 Azure 反馈 以实现适当的运算符
I urge you to vote for the Azure Feedback to implement a proper operator
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