SQL Server에서 중첩 된 사례 명령문 논리를 수행하는 가장 좋은 방법

SQL Server에서 중첩 된 사례 명령문 논리를 수행하는 가장 좋은 방법

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SQL 쿼리를 작성 중입니다. 여기서 많은 열에 따라 반환되는 몇 가지 열을 계산해야합니다.

나는 현재 중첩 된 case 문을 사용하고 있지만 지저분 해집니다. 더 나은 (보다 체계적이고 읽기 쉬운) 방법이 있습니까?

(저는 Microsoft SQL Server 2005를 사용하고 있습니다)

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간단한 예 :

SELECT
    col1,
    col2,
    col3,
    CASE
        WHEN condition 
        THEN
            CASE
                WHEN condition1 
                THEN
                    CASE 
                        WHEN condition2
                        THEN calculation1
                        ELSE calculation2
                    END
                ELSE
                    CASE 
                        WHEN condition2
                        THEN calculation3
                        ELSE calculation4
                    END
            END
        ELSE 
            CASE 
                WHEN condition1 
                THEN 
                    CASE
                        WHEN condition2 
                        THEN calculation5
                        ELSE calculation6
                    END
                ELSE
                    CASE
                        WHEN condition2 
                        THEN calculation7
                        ELSE calculation8
                    END
            END            
    END AS 'calculatedcol1',
    col4,
    col5 -- etc
FROM table

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다음과 같은 일종의 COALESCE 트릭을 시도해 볼 수 있습니다.

콜라이 선택 (
  조건 1 다음 계산 1 다른 경우 NULL 종료시,
  조건 2 다음에 계산 2 다른 경우 NULL 종료
  기타...
)

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모든 케이스를 하나로 포장하십시오.

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SELECT
    col1,
    col2,
    col3,
    CASE
        WHEN condition1 THEN calculation1 
        WHEN condition2 THEN calculation2
        WHEN condition3 THEN calculation3
        WHEN condition4 THEN calculation4
        WHEN condition5 THEN calculation5
        ELSE NULL         
    END AS 'calculatedcol1',
    col4,
    col5 -- etc
FROM table

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상황을 피하기 위해 여러 조건을 결합 할 수 있습니다.

CASE WHEN condition1 = true AND condition2 = true THEN calculation1 
     WHEN condition1 = true AND condition2 = false 
     ELSE 'what so ever' END,

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나는 내장 CASE 표현을 제한하면서 개인적으로 이렇게합니다. 또한 무슨 일이 일어나고 있는지 설명하기 위해 의견을 달았습니다. 너무 복잡하면 기능으로 분리하십시오.

SELECT
    col1,
    col2,
    col3,
    CASE WHEN condition THEN
      CASE WHEN condition1 THEN
        CASE WHEN condition2 THEN calculation1
        ELSE calculation2 END
      ELSE
        CASE WHEN condition2 THEN calculation3
        ELSE calculation4 END
      END
    ELSE CASE WHEN condition1 THEN 
      CASE WHEN condition2 THEN calculation5
      ELSE calculation6 END
    ELSE CASE WHEN condition2 THEN calculation7
         ELSE calculation8 END
    END AS 'calculatedcol1',
    col4,
    col5 -- etc
FROM table

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Here's a simple solution to the nested "Complex" case statment: --Nested Case Complex Expression

select  datediff(dd,Invdate,'2009/01/31')+1 as DaysOld, 
    case when datediff(dd,Invdate,'2009/01/31')+1 >150 then 6 else
        case when datediff(dd,Invdate,'2009/01/31')+1 >120 then 5 else 
            case when datediff(dd,Invdate,'2009/01/31')+1 >90 then 4 else 
                case when datediff(dd,Invdate,'2009/01/31')+1 >60 then 3 else 
                    case when datediff(dd,Invdate,'2009/01/31')+1 >30 then 2 else 
                        case when datediff(dd,Invdate,'2009/01/31')+1 >30 then 1 end 
                    end
                end
            end
        end
    end as Bucket
from rm20090131atb

Just make sure you have an end statement for every case statement

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a user-defined function may server better, at least to hide the logic - esp. if you need to do this in more than one query

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We can combine multiple conditions together to reduce the performance overhead.

Let there are three variables a b c on which we want to perform cases. We can do this as below:

CASE WHEN a = 1 AND b = 1 AND c = 1 THEN '1'
     WHEN a = 0 AND b = 0 AND c = 1 THEN '0'
ELSE '0' END,

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I went through this and found all the answers super cool, however wants to add to answer given by @deejers

    SELECT
    col1,
    col2,
    col3,
    CASE
        WHEN condition1 THEN calculation1 
        WHEN condition2 THEN calculation2
        WHEN condition3 THEN calculation3
        WHEN condition4 THEN calculation4
        WHEN condition5 THEN calculation5         
    END AS 'calculatedcol1',
    col4,
    col5 -- etc
FROM table

you can make ELSE optional as its not mandatory, it is very helpful in many scenarios.

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This example might help you, the picture shows how SQL case statement will look like when there are if and more than one inner if loops

참고URL : https://stackoverflow.com/questions/505747/best-way-to-do-nested-case-statement-logic-in-sql-server