Question

我想知道如何使用基于WHERE或CASE的其他IF子句。我更喜欢CASE，因为声明的其余部分很复杂，我不喜欢这种复杂性在两个地方只有一点点差异的想法。但是，我知道案例仅用于值。我在下面复制了我的问题的简单版本。

基本上，我有三张桌子。第一个包含主信息（MasterTable）。第二个包含属于主表的一对多关系（表1）。第三个是选择器列表，指示表1中的哪些记录将在此实例中使用。我想要Table2的最新记录来驱动从Table1中选择的内容，优先使用OrderNum上的SubID。

MasterTable | MasterID，OtherInfo

表1 | T1UniqueId，MasterID，SubID，Text，OrderNum

表2 | T2UniqueId，MasterID，SubID，OrderNum，Date

SELECT MasterID, OtherInfo, SubID
FROM MasterTable
OUTER APPLY(
    SELECT TOP 1 SubID FROM Table1
    WHERE Table1.MasterID=MasterTable.MasterID
    CASE
        WHEN 
        (
            SELECT TOP 1 SubID FROM Table2 
            WHERE Table2.MasterID=MasterTable.MasterID 
            ORDER BY Date DESC
        ) Is NULL
        THEN Table1.OrderNum=
            (
                SELECT TOP 1 OrderNum 
                FROM Table2
                WHERE Table2.MasterId=MasterTable.MasterId
                ORDER BY Date DESC
            )
        ELSE Table1.SubId=
            (
                SELECT TOP 1 SubId
                FROM Table2
                WHERE Table2.MasterId=MasterTable.MasterId
                ORDER BY Date DESC
            )
    END

) SubData

Answer 1

快速重写一次将导致以下结果：

IF ((SELECT TOP 1 SubID FROM Table2 WHERE Table2.MasterID=MasterTable.MasterID ORDER BY Date DESC) IS NULL)
BEGIN
    SELECT
            MasterID, OtherInfo, SubID
        FROM MasterTable
        OUTER APPLY(
            SELECT TOP 1 SubID FROM Table1
            WHERE
                Table1.MasterID=MasterTable.MasterID
                AND Table1.OrderNum =
                (
                    SELECT TOP 1 OrderNum 
                    FROM Table2
                    WHERE Table2.MasterId=MasterTable.MasterId
                    ORDER BY Date DESC
                )
        ) SubData
END
ELSE
BEGIN
    SELECT
            MasterID, OtherInfo, SubID
        FROM MasterTable
        OUTER APPLY(
            SELECT TOP 1 SubID FROM Table1
            WHERE
                Table1.MasterID=MasterTable.MasterID
                AND Table1.SubId=
                (
                    SELECT TOP 1 SubId
                    FROM Table2
                    WHERE Table2.MasterId=MasterTable.MasterId
                    ORDER BY Date DESC
                )
        ) SubData
END

但正如你所指出的那样让它看起来很难看，因为你现在在两个地方都有这种复杂性......

我想你也可以用这种方式表达它（未经测试，但这应该将你的复杂逻辑保存在一个地方）：

SELECT
        MasterID, OtherInfo, SubID
    FROM MasterTable
    OUTER APPLY(
        SELECT TOP 1 SubID FROM Table1
        WHERE Table1.MasterID=MasterTable.MasterID
        AND
        (
            (
                (
                    SELECT
                            TOP 1 SubID
                        FROM Table2 
                        WHERE Table2.MasterID=MasterTable.MasterID 
                        ORDER BY Date DESC
                ) IS NULL
                AND
                Table1.OrderNum =
                (
                    SELECT TOP 1 OrderNum 
                    FROM Table2
                    WHERE Table2.MasterId=MasterTable.MasterId
                    ORDER BY Date DESC
                )
            )
            OR
            (
                Table1.SubId =
                (
                    SELECT
                            TOP 1 SubId
                        FROM Table2
                        WHERE Table2.MasterId=MasterTable.MasterId
                        ORDER BY Date DESC
                )
            )
        )
    ) SubData

Answer 2

如果Table1和Table2中的SubID和OrderNum相同，您可以使用嵌套select语句的简单查询：

select m.MasterID, m.OtherInfo, (
  select top 1 coalesce(t2.SubID, t2.OrderNum) from Table2 t2
  where t2.MasterID = m.MasterID order by date desc
) as SubID
from MasterTable m;

使用两个不同的where子句

2 个答案: