以多种方式在 SQL 查询中多次使用复杂语句

Posted 2023-04-14

【中文标题】以多种方式在 SQL 查询中多次使用复杂语句

【英文标题】：Using complex statements in SQL queries multiple times in multiple ways

【发布时间】：2015-05-04 07:45:48

【问题描述】：

我正在设计一个 Microsoft SQL Server 2008 上的 View，它应该建立在相当多的业务逻辑之上，这意味着有很多 CASE WHEN THEN ELSE 语句。问题是，一个CASE 语句的结果通常在查询的其他地方需要，例如函数、连接和其他情况。这使得代码非常臃肿、难以阅读和维护。

以下是此类视图的示例（字段和功能仅是说明性的）：

SELECT

    -- Random complicated case
    CASE
        WHEN A=B AND CAST(C AS int)=D AND DATEDIFF(DAY,E,F) > 5
        THEN D-A
        WHEN A<>B AND CAST(C AS int)=D AND DATEDIFF(DAY,E,F) > 5
        THEN D-A - 5
        WHEN A=B AND CAST(C AS int)<>D AND DATEDIFF(DAY,E,F) > 5
        THEN D-A - 10
        WHEN A<>B AND CAST(C AS int)<>D AND DATEDIFF(DAY,E,F) > 5
        THEN D-A - 15
    END AS ComplicatedCase

    -- Use of that same complicated case in another case
    CASE
        WHEN CASE
                WHEN A=B AND CAST(C AS int)=D AND DATEDIFF(DAY,E,F) > 5
                THEN D-A
                WHEN A<>B AND CAST(C AS int)=D AND DATEDIFF(DAY,E,F) > 5
                THEN D-A - 5
                WHEN A=B AND CAST(C AS int)<>D AND DATEDIFF(DAY,E,F) > 5
                THEN D-A - 10
                WHEN A<>B AND CAST(C AS int)<>D AND DATEDIFF(DAY,E,F) > 5
                THEN D-A - 15
             END > 300
        THEN CASE
                WHEN A=B AND CAST(C AS int)=D AND DATEDIFF(DAY,E,F) > 5
                THEN D-A
                WHEN A<>B AND CAST(C AS int)=D AND DATEDIFF(DAY,E,F) > 5
                THEN D-A - 5
                WHEN A=B AND CAST(C AS int)<>D AND DATEDIFF(DAY,E,F) > 5
                THEN D-A - 10
                WHEN A<>B AND CAST(C AS int)<>D AND DATEDIFF(DAY,E,F) > 5
                THEN D-A - 15
             END 
        ELSE NULL
    END AS AnotherCase

FROM SomeTable AS T
    -- Complicated case in join
    INNER JOIN AnotherTable AS AT
        ON  AT.ID = CASE
                        WHEN A=B AND CAST(C AS int)=D AND DATEDIFF(DAY,E,F) > 5
                        THEN D-A
                        WHEN A<>B AND CAST(C AS int)=D AND DATEDIFF(DAY,E,F) > 5
                        THEN D-A - 5
                        WHEN A=B AND CAST(C AS int)<>D AND DATEDIFF(DAY,E,F) > 5
                        THEN D-A - 10
                        WHEN A<>B AND CAST(C AS int)<>D AND DATEDIFF(DAY,E,F) > 5
                        THEN D-A - 15
                    END

代码很长，不可读，如果需要更改业务逻辑，我更有可能忘记更改。

我想到的明显解决方案是在子选择中添加案例，通过INNER JOIN 或CROSS APPLY 加入，如下所示：

-- Solution by CROSS APPLY sub-select
SELECT
    T1.ComplicatedCase,
    CASE
        WHEN ComplicatedCase > 300
        THEN ComplicatedCase
        ELSE NULL
    END AS AnotherCase

FROM SomeTable AS T
    CROSS APPLY
        (
        SELECT
            CASE
                WHEN A=B AND CAST(C AS int)=D AND DATEDIFF(DAY,E,F) > 5
                THEN D-A
                WHEN A<>B AND CAST(C AS int)=D AND DATEDIFF(DAY,E,F) > 5
                THEN D-A - 5
                WHEN A=B AND CAST(C AS int)<>D AND DATEDIFF(DAY,E,F) > 5
                THEN D-A - 10
                WHEN A<>B AND CAST(C AS int)<>D AND DATEDIFF(DAY,E,F) > 5
                THEN D-A - 15
            END AS ComplicatedCase
        FROM SomeTable AS ST
        WHERE T.ID = ST.ID
        ) AS T1

    INNER JOIN AnotherTable AS AT ON AT.ID = T1.ID

...而且效果很好，但我不知道查询性能是否不会大规模受到影响（因为我不太了解引擎如何在内部处理这些东西）。

对于如何处理此类复杂语句，您有任何最佳实践吗？如果它在一个简单的连接子选择中，它甚至对 SQL 引擎有影响吗？

PS：我的case语句通常由几个函数组成，主要是datetime的处理和转换。

【参考方案1】：

我认为你不必CROSS APPLY你的SomeTable两次。

这应该可行：

SELECT T1.ComplicatedCase
    , CASE
        WHEN ComplicatedCase > 300 THEN ComplicatedCase
        ELSE NULL
    END AS AnotherCase
FROM SomeTable AS T
CROSS APPLY (
    SELECT DATEDIFF(DAY, T.E, T.F)
    ) AS TT(DayDiff)
CROSS APPLY (
    SELECT CASE
            WHEN T.A = T.B  AND CAST(T.C AS INT) = T.D  AND TT.DayDiff > 5 THEN D - A
            WHEN T.A <> T.B AND CAST(T.C AS INT) = T.D  AND TT.DayDiff > 5 THEN D - A - 5
            WHEN T.A = T.B  AND CAST(T.C AS INT) <> T.D AND TT.DayDiff > 5 THEN D - A - 10
            WHEN T.A <> T.B AND CAST(T.C AS INT) <> T.D AND TT.DayDiff > 5 THEN D - A - 15
        END
    ) AS T1(ComplicatedCase)
INNER JOIN AnotherTable AS AT
    ON AT.ID = T1.ComplicatedCase;

CROSS APPLY 允许您创建计算值并在JOINS、WHERE 语句等中使用它们。它使代码更具可读性，并且不应该为您花费额外的资源。

如果有什么不清楚或不符合您的标准 - 请告诉我。

【讨论】：

嗨@Evaldas，除了正确的表引用之外，我认为您的解决方案和我的解决方案没有太大区别。但我喜欢只为 DATEDIFF 使用 CROSS APPLY 的想法。这对 SQL 引擎有影响还是只是为了可读性？另外，将别名分配给带括号的子选择是什么意思？像这样：AS TT(DayDiff)...我以前没见过这个。谢谢！

嘿@MarekStejskal，我看到了不同之处，您在不需要时在SomeTable 之间进行了两次引用（至少对我来说看起来是这样），这是额外费用，请参见此处：prntscr.com/71csl1。它不应该对 SQL Server 引擎产生任何显着影响，它只是保持可读性。对于您的第二个问题 - 这只是分配别名的另一种方式。我习惯于为CROSS APPLY 或OUTER APPLY 这样做。