Question

我正在尝试使用下面的查询（完美地工作），作为UPDATE语句的子查询，并且需要在该UPDATE语句中引用值CORRECT_DATE。如果我可以在此查询中成功地选择该值，那么将其传递给UPDATE将很容易，但我似乎无法选择下面的相关子查询值并在不等式中使用它。

SELECT A.NAME, MAX(A.[START])
FROM dbo.TANK A
WHERE A.[CURRENT] = 0 AND A.[END] < (SELECT B.[START] AS CORRECT_DATE
                                          FROM dbo.TANK B
                         WHERE B.[CURRENT] = 1
                         AND A.NAME = B.NAME
                         AND A.TYPE = B.TYPE
                              AND A.COVER = B.COVER)

我尝试过以下操作，但它不识别WHERE子句中的别名：

SELECT A.NAME, MAX(A.[START]), (SELECT B.[START] AS CORRECT_DATE
                                          FROM dbo.TANK B
                         WHERE B.[CURRENT] = 1
                         AND A.NAME = B.NAME
                         AND A.TYPE = B.TYPE
                              AND A.COVER = B.COVER) as subvalue
FROM dbo.TANK A
WHERE A.[CURRENT] = 0 AND A.[END] < subvalue

Answer 1

为什么不使用加入？相关的子查询是应尽可能避免的性能杀手。

SELECT A.NAME
    , MAX(A.[START])
    , B.[START]                                           
FROM dbo.TANK A 
JOIN dbo.TANK B
    ON A.NAME = B.NAME 
    AND A.TYPE = B.TYPE
    AND A.COVER = B.COVER AND B.[CURRENT] = 1 
WHERE A.[CURRENT] = 0 AND A.[END] < B.[START]
     GROUP BY A.NAME, B.[START];

在主查询中使用相关子查询值

1 个答案: