Question

我不太明白为什么这两个不同的代码示例会返回不同的值。

以某种方式不正确但工作语法，返回错误结果，例如，当通过两个相等的值进行比较时，它返回0：

(SELECT CASE 
    WHEN 
       SUM(V.IsCompatible) OVER 
          (PARTITION BY ComputerName, UserID) = ApplicationCount 
    THEN 1 ELSE 0 END
) AS CompatibleUser

下面的一个返回正确的值，即。当两个相等的值进行比较时1。

(CASE 
    WHEN 
       SUM(V.IsCompatible) OVER 
         (PARTITION BY ComputerName, UserID) = ApplicationCount 
    THEN 1 ELSE 0 END
) AS CompatibleUser

甚至更简单：

(SELECT CASE 
    WHEN 
       X = Y 
    THEN 1 ELSE 0 END
) AS Result

X = 22且Y = 22 =＆gt;结果= 0

(CASE 
    WHEN 
       X = Y 
    THEN 1 ELSE 0 END
) AS Result

X = 22且Y = 22 =＆gt;结果= 1

我理解应用正确的语法很重要，我知道T-SQL中的SELECT CASE语法，但我不明白如何评估第一个代码示例并带来意想不到的结果。

更新：在其上下文中的完整查询

select userapplication.username, computerdetails.computername, sum(userapplication.iscompatible) over (partition by computerdetails.computername, userapplication.userid) as compatiblecount, userapplication.applicationcount, ( case when sum(userapplication.iscompatible) over (partition by computerdetails.computername, userapplication.userid) <> userapplication.applicationcount then 0 else 1 end ) as usercomputeriscompatible from computerdetails right outer join usercomputer on computerdetails.computerid = usercomputer.computerid right outer join userapplication on usercomputer.gebruikerid = userapplication.userid

所以userComputerIsCompatible是有问题的结果

Answer 1

我认为这种行为的原因是下一个：像(SELECT ...)这样的表达式被认为是子查询，即使它们没有FROM子句。假设这些（错误）“子查询”的数据源仅是当前行。因此，(SELECT expression)被解释为(SELECT expression FROM current_row)而(SELECT SUM(iscompatible)OVER(...))被解释为(SELECT SUM(iscompatible)OVER(current_row))。

参数：分析(SELECT SUM(IsWeb) OVER(PARTITION BY OrderDate) [FROM current_row])表达式的执行计划 enter image description here

我在Clustered Index Scan和Segment（Stream Aggregate）运算符之前看到Constant Scan (Scan an internal table of constants)运算符而不是[Expr1007] = Scalar Operator(SUM(@OrderHeader.[IsWeb] as [h].[IsWeb]))运算符。此内部表（Constant Scan）由当前行构建。

示例（使用SQL2005SP3和SQL2008测试）：

DECLARE @OrderHeader TABLE
(
     OrderHeaderID  INT IDENTITY PRIMARY KEY
    ,OrderDate      DATETIME NOT NULL
    ,IsWeb          TINYINT NOT NULL --or BIT
);
INSERT  @OrderHeader 
SELECT  '20110101', 0
UNION ALL 
SELECT  '20110101', 1
UNION ALL
SELECT  '20110101', 1
UNION ALL 
SELECT  '20110102', 1
UNION ALL
SELECT  '20110103', 0
UNION ALL
SELECT  '20110103', 0;

SELECT  *
        ,SUM(IsWeb) OVER(PARTITION BY OrderDate) SumExpression_1
FROM    @OrderHeader h
ORDER BY h.OrderDate;

SELECT  *
        ,(SELECT SUM(IsWeb) OVER(PARTITION BY OrderDate)) SumWithSubquery_2
FROM    @OrderHeader h
ORDER BY h.OrderDate;

结果：

OrderHeaderID OrderDate               IsWeb SumExpression_1
------------- ----------------------- ----- ---------------
1             2011-01-01 00:00:00.000 0     2
2             2011-01-01 00:00:00.000 1     2
3             2011-01-01 00:00:00.000 1     2
4             2011-01-02 00:00:00.000 1     1
5             2011-01-03 00:00:00.000 0     0
6             2011-01-03 00:00:00.000 0     0

OrderHeaderID OrderDate               IsWeb SumWithSubquery_2
------------- ----------------------- ----- -----------------
1             2011-01-01 00:00:00.000 0     0
2             2011-01-01 00:00:00.000 1     1
3             2011-01-01 00:00:00.000 1     1
4             2011-01-02 00:00:00.000 1     1
5             2011-01-03 00:00:00.000 0     0
6             2011-01-03 00:00:00.000 0     0

Answer 2

我尝试了你的代码，我得到了两个查询相同的结果。这是我试过的代码：

DECLARE @X INT = 22
DECLARE @Y INT = 22

SELECT (SELECT CASE 
    WHEN 
       @X = @Y 
    THEN 1 ELSE 0 END
) AS Result

SELECT (CASE 
    WHEN 
       @X = @Y 
    THEN 1 ELSE 0 END
) AS Result

结果是 1 和 1

我是在SQL Server 2008 R2上运行的

在SQL中选择CASE与CASE

2 个答案: