Question

我正在编写一个“简单”的SQL函数来选择数据，具体取决于WHERE子句中的参数：

DECLARE @csp AS CHAR(1) = 'F'

SELECT  [NUMERO_DOSSIER]
FROM    [dbo].[Dossier]
WHERE   
    [CSP] IN
    (
        CASE @csp
            WHEN 'F' THEN
                (SELECT [CSP] FROM [dbo].[CSP_1])
            WHEN 'N' THEN
                (SELECT [CSP] FROM [dbo].[CSP_2])
            END
    )

根据SQL Server 2012，此查询没有语法错误。但运行它会返回此错误：

Msg 512，Level 16，State 1，Line 2子查询返回的值超过1。当子查询遵循=，！=，＆lt;，＆lt; =，＆gt;，＆gt; =或子查询用作表达式时，不允许这样做。

根据我的理解，这里的问题是：CASE函数返回标量，并且不能返回表。

如何重写它以使其有效？

Answer 1

不要在CASE子句中使用WHERE - 它通常会使逻辑更复杂。只需将逻辑扩展为布尔逻辑：

WHERE ( @csp = 'F' AND [CSP] IN (SELECT [CSP] FROM [dbo].[CSP_1]) ) OR
      ( @csp = 'N' AND [CSP] IN (SELECT [CSP] FROM [dbo].[CSP_2]) )

Answer 2

一种方法是使用UNION ALL子查询：

SELECT  [NUMERO_DOSSIER]
FROM    [dbo].[Dossier]
WHERE   
    [CSP] IN
    (
        SELECT [CSP] 
        FROM [dbo].[CSP_1]
        WHERE @csp = 'F'
        UNION ALL
        SELECT [CSP] 
        FROM [dbo].[CSP_2]
        WHERE @csp = 'N'
    );

Answer 3

使用两个CTE和两个JOIN操作：

DECLARE @csp AS CHAR(1) = 'F'

;WITH Table1 AS (
    SELECT [CSP] 
    FROM [dbo].[CSP_1]
), Table2 AS (
    SELECT [CSP] 
    FROM [dbo].[CSP_2]
)
SELECT  [NUMERO_DOSSIER]
FROM  [dbo].[Dossier] AS d
JOIN Table1 AS t1 ON @csp = 'F' AND d.CSP = t1.CSP
JOIN Table2 AS t2 ON @csp = 'N' AND d.CSP = t2.CSP

修改

看到@ Dan的回答后，这可能更干净了：

;WITH Table1 AS ( SELECT [CSP] FROM [dbo].[CSP_1] WHERE @csp = 'F' ), Table2 AS ( SELECT [CSP] FROM [dbo].[CSP_2] WHERE @csp = 'N' ) SELECT [NUMERO_DOSSIER] FROM [dbo].[Dossier] AS d JOIN Table1 AS t1 ON d.CSP = t1.CSP JOIN Table2 AS t2 ON d.CSP = t2.CSP

根据where子句中的参数从表中选择

3 个答案: