Sql IN中的Sql Case语句 [英] Sql Case statement within Sql IN

问题描述

我有一个sql语句，该语句具有如下所示的where条件

I have a sql statement which have a where condition like below

WHERE (TABLE1.Field=@Id OR (@Id=0))

当@Id值为零或更大时，此方法正常工作

This works fine when @Id values is zero or more

现在，当@Id值变为0时，我需要匹配多个值.

Now I need to match multiple values when @Id value become 0.

我尝试了以下类似的操作，但不起作用.我知道我可以在这里使用IF-ELSE，但是想知道是否可以使用case statemet

I tried something like below which doesn't work. I know that I can use IF-ELSE in here, but wonder whether this can be achieve using case statemet

WHERE TABLE1.Field IN (CASE WHEN @Id>0 THEN @Id ELSE (6,16,18) END)

推荐答案

您可以使用OR:

WHERE   (@Id > 0 AND Table1.Field = @Id)
OR      (@Id = 0 AND Table1.Field IN (6,16,18))

但是，我建议使用(正如您所说的)IF/ELSE，当将两个这样的条件混合在一起时，您通常会强制执行次优计划.例如，在您的示例中，您可以将其简化为如下所示的模式:

However, I would advise using (as you have said) IF/ELSE, when mashing together two conditions like this you can often force suboptimal plans. e.g In your example you could simplify this to a schema as follows:

CREATE TABLE T
(   ID INT IDENTITY(1, 1) NOT NULL PRIMARY KEY, 
    Field INT NOT NULL, 
    SomeOtherField INT NULL
);
GO
INSERT T  (Field)
SELECT  Number
FROM    Master..spt_values
        CROSS JOIN (VALUES (1), (2), (3)) t (A)
WHERE   Type = 'P'
GO
CREATE NONCLUSTERED INDEX IX_T_Field ON T (Field) INCLUDE (SomeOtherField);

这只是用数字0-2047填充其中一列，每列重复4次(仅用于某些示例数据).然后，如果我创建了两个过程，则一个使用"IF/ELSE"，一个结合了上述标准:

This simply populates one of the columns with the numbers 0-2047 repeated 4 times each (just for some example data). Then If I create two procedures, one that uses 'IF/ELSE' one that combines the criteria as above:

CREATE PROCEDURE dbo.Test @ID INT
AS
    SELECT  ID, Field, SomeOtherField
    FROM    T
    WHERE   (@Id > 0 AND T.Field = @Id)
    OR      (@Id = 0 AND T.Field IN (6,16,18))

GO
CREATE PROCEDURE dbo.Test2 @ID INT
AS
    IF @ID = 0
        SELECT  ID, Field, SomeOtherField
        FROM    T
        WHERE   T.Field IN (6, 16, 18)
    ELSE
        SELECT  ID, Field, SomeOtherField
        FROM    T
        WHERE   T.Field = @Id
GO

由于查询的编译只会发生一次(除非您另外明确地声明)，所以优化程序将不会选择其他计划，具体取决于您是向过程传递0还是将ID> 0传递给过程，因此以下两项:/p>

Since compilation of queries will only happen once (unless you explicitly say otherwise), the optimiser will not pick a different plan depending on whether you pass 0 or pass an ID > 0 to the procedure, so both of the following:

EXECUTE dbo.Test 0;
EXECUTE dbo.Test 1;

会给出这个计划:

第二个过程能够更好地估计最佳执行计划，因此运行以下命令:

The second procedure is able to estimate the best execution plan much better so running this:

EXECUTE dbo.Test2 0;
EXECUTE dbo.Test2 1;

给出以下计划:

现实世界中的示例显然会有所不同，我特意构建了一个证明我的观点的示例.使用IF/ELSE来复制大量代码要花些力气，但这通常是值得的.

Real world examples will obviously vary, and I have deliberately constructed an example that proves my point. It is slightly more effort to duplicate a lot of code by using IF/ELSE, but it is often worth it.

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