從表中選擇，具體取決於where子句中的參數

Question

我正在寫一個“簡單的” SQL函數，該函數根據WHERE子句中的參數選擇數據：

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
    )

根據SQL Server 2012，此查詢沒有語法錯誤。但是運行查詢會返回以下錯誤：

消息512，級別16，狀態1，第2行子查詢返回的值超過1。 當子查詢遵循=，！=，<，<=，>，> =或將子查詢用作表達式時，不允許這樣做。

據我了解，這里的問題是： CASE函數返回標量，而不能返回表。

我該如何重寫才能使其正常工作？

Answer 1

不要在WHERE子句中使用CASE －通常會使邏輯更加復雜。 只需將邏輯擴展為布爾邏輯即可：

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

一種方法是使用UNION ALL子查詢：

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

使用兩個CTE和兩個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

編輯：

看到@Dan的答案后，這可能會更干凈：

;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