查询时的Sql案例答案

【问题标题】：Sql Case When query查询时的Sql案例
【发布时间】：2013-06-11 08:15:22
【问题描述】：

我发现“CASE WHEN”语句对我的查询非常有用。但这是我的查询（仅选择）：

SELECT  dbo.ARCHIVE.SYSTEM_KEY AS PROTOCOLLO, 
        CASE dbo.ARCHIVEDEST.ERR_ID WHEN 0 THEN 'OK' ELSE 'KO' END AS ESITO, 
        CASE WHEN dbo.ARCHIVEDEST.XMODE IN ('R', 'K', 'H') THEN 'RX' ELSE 'TX' END AS 'T/R', 
        CASE 'T/R' WHEN 'TX' THEN CONTACTORIGIN.address ELSE CONTACTDESTINATION.address END AS Utente,

问题出在第三个 Case 语句上，因为不评估先前的“T/R”查询（对于所有记录，它返回“CONTACTORIGIN.address”。有可能这样做吗？还是我走错路了？

【问题讨论】：

标签： sql sql-server sql-server-2008 tsql select

【解决方案1】：

使用子查询定义T/R别名；

SELECT  *
,       CASE 'T/R' WHEN 'TX' THEN CONTACTORIGIN.address ELSE CONTACTDESTINATION.address 
            END AS Utente
FROM    (
        SELECT  CASE dbo.ARCHIVEDEST.ERR_ID WHEN 0 THEN 'OK' ELSE 'KO' END AS ESITO
        ,       CASE WHEN dbo.ARCHIVEDEST.XMODE IN ('R', 'K', 'H') THEN 'RX' ELSE 'TX' 
                    END AS 'T/R'
        FROM    dbo.ARCHIVE.SYSTEM_KEY AS PROTOCOLLO
        ) as SubQueryAlias

【讨论】：

【解决方案2】：

你必须重复测试：

SELECT  dbo.ARCHIVE.SYSTEM_KEY AS PROTOCOLLO, 
        CASE dbo.ARCHIVEDEST.ERR_ID WHEN 0 THEN 'OK' ELSE 'KO' END AS ESITO, 
        CASE WHEN dbo.ARCHIVEDEST.XMODE IN ('R', 'K', 'H') THEN 'RX' ELSE 'TX' END AS 'T/R', 
        CASE CASE WHEN dbo.ARCHIVEDEST.XMODE NOT IN ('R', 'K', 'H') THEN CONTACTORIGIN.address ELSE CONTACTDESTINATION.address END AS Utente,

或者你可以做一个子查询：

SELECT PROTOCOLLO, ESITO, [T/R],
    CASE WHEN [T/R] = 'TX' THEN CONTACTORIGIN.address ELSE CONTACTDESTINATION.address END AS Utente
FROM
(SELECT  dbo.ARCHIVE.SYSTEM_KEY AS PROTOCOLLO, 
        CASE dbo.ARCHIVEDEST.ERR_ID WHEN 0 THEN 'OK' ELSE 'KO' END AS ESITO, 
        CASE WHEN dbo.ARCHIVEDEST.XMODE IN ('R', 'K', 'H') THEN 'RX' ELSE 'TX' END AS [T/R], 
) s

【讨论】：

也可以考虑使用子选择或视图。
只是在编辑我的答案添加子查询，好点:)

【解决方案3】：

如果您不想重复您的案例陈述，您有两个选择，第一个是将其中一个移动到子查询中，即

SELECT  PROTOCOLLO,
        ESITO,
        [T/R],
        CASE [T/R] WHEN 'TX' THEN CONTACTORIGIN.address ELSE CONTACTDESTINATION.address END AS Utente
FROM    (   SELECT  dbo.ARCHIVE.SYSTEM_KEY AS PROTOCOLLO, 
                    CASE dbo.ARCHIVEDEST.ERR_ID WHEN 0 THEN 'OK' ELSE 'KO' END AS ESITO, 
                    CASE WHEN dbo.ARCHIVEDEST.XMODE IN ('R', 'K', 'H') THEN 'RX' ELSE 'TX' END AS 'T/R'
            FROM    ....
        ) SubQuery

或者您可以将您的 CASE 声明移到 CROSS APPLY 中：

SELECT  dbo.ARCHIVE.SYSTEM_KEY AS PROTOCOLLO, 
        CASE dbo.ARCHIVEDEST.ERR_ID WHEN 0 THEN 'OK' ELSE 'KO' END AS ESITO, 
        'T/R', 
        CASE [T/R] WHEN 'TX' THEN CONTACTORIGIN.address ELSE CONTACTDESTINATION.address END AS Utente
FROM    ...
        CROSS APPLY
        (   SELECT  CASE WHEN dbo.ARCHIVEDEST.XMODE IN ('R', 'K', 'H') THEN 'RX' ELSE 'TX' END AS 'T/R'
        ) tx

【讨论】：

【解决方案4】：

当您在 SELECT 子句中计算列的值时，您不能访问在同一 SELECT 子句中计算的任何其他计算值 - 因为（根据 SQL 标准）列是被并行计算（它们可能不是，但假设某些实现将这样做），因此计算值尚不可用。

您可以将“较早”的计算放在子查询或 CTE 中：

SELECT *,
    CASE [T/R] WHEN 'TX' THEN CONTACTORIGIN.address ELSE CONTACTDESTINATION.address END AS Utente
FROM (
    SELECT  dbo.ARCHIVE.SYSTEM_KEY AS PROTOCOLLO, 
        CASE dbo.ARCHIVEDEST.ERR_ID WHEN 0 THEN 'OK' ELSE 'KO' END AS ESITO, 
        CASE WHEN dbo.ARCHIVEDEST.XMODE IN ('R', 'K', 'H') THEN 'RX' ELSE 'TX' END AS [T/R], 

    ...
) t

【讨论】：