Question

我在View上设计一个Microsoft SQL Server 2008，它应该基于很多业务逻辑构建，这意味着有很多CASE WHEN THEN ELSE语句。问题是，查询中的其他位置经常需要一个CASE语句的结果，例如函数，联接和其他情况。这使得代码非常臃肿，难以阅读并且难以维护。

以下是此类视图的示例（字段和函数仅用于说明）：

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，我更可能忘记更改。

我遇到的一个明显的解决方案就是将子案例放在子选择中，或者由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

...并且工作正常，但我不知道查询性能是否会受到大规模影响（因为我不太了解引擎如何在内部处理这些内容）。

您是否有关于如何处理此类复杂语句的最佳实践？如果它是一个简单的连接子选择，它对SQL引擎是否重要？

PS：我的案例陈述通常由几个函数组成，主要是datetime处理和转换。

Answer 1

我不认为你必须var btns=''; var category = ["fur_", "fts_", "fas_"]; for(i = 1; i < category.length; i++){ btns +='<button type="button" class='+category[i]+' id= "myBtn'+i+'">Button</button>'; } document.getElementById('div').innerHTML = btns; var button = document.getElementById('myBtn1'); button.addEventListener('click', function () { alert('Clicked'); }, false); CROSS APPLY两次。

这应该有效：

SomeTable

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;允许您创建计算值并在CROSS APPLY，JOINS语句中使用它们，等等。它使代码更具可读性，并且不应为您节省额外资源。

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在SQL查询中以多种方式多次使用复杂语句

1 个答案: