datediff帶有選擇條件的條件

Question

Sql Server 2008 Datediff帶有select語句中的條件。

我有下面的查詢，需要幫助理解爲什麼我得到這些結果和我的問題的最佳解決方案。

SELECT DISTINCT l.program, l.id 
    ,(DATEDIFF(DAY, B.FIRST_DT , B.FIRST_second_DT))days_between1 
    ,(DATEDIFF(DAY, C.FIRST_DT , C.FIRST_second_DT))days_between2  
FROM l 
LEFT JOIN l B ON L.id = B.id AND L.SERVICE_DATE = B.SERVICE_DATE 
       AND B.FIRST_DT IS NOT NULL AND B.FIRST_DT IS NOT NULL 
       AND l.IS ='0' AND L.FIRST_V_DT IS NOT NULL 
LEFT JOIN l C ON L.id = C.id AND L.SERVICE_DATE = C.SERVICE_DATE 
       AND C.FIRST_DT IS NOT NULL 
       AND C.FIRST_A_DT IS NOT NULL AND L.IS ='1' 
       AND L.FIRST_DT IS NOT NULL 
WHERE 1=1 

我得到兩條線

Program id daysbetween1  daysbetween2 
blue  1  null    3 
blue  1   4    null 

如何獲取一條線，如下圖所示，爲什麼我會收到兩行呢？

Program id daysbetween1  daysbetween2 
blue  1  4    3 

Answer 1

你可以試試這個方法：

SELECT l.program, l.id 
    ,MAX((DATEDIFF(DAY, B.FIRST_DT , B.FIRST_second_DT))) days_between1 
    ,MAX((DATEDIFF(DAY, C.FIRST_DT , C.FIRST_second_DT))) days_between2  
FROM l 
LEFT JOIN l B ON L.id = B.id AND L.SERVICE_DATE = B.SERVICE_DATE 
       AND B.FIRST_DT IS NOT NULL AND B.FIRST_DT IS NOT NULL 
       AND l.IS ='0' AND L.FIRST_V_DT IS NOT NULL 
LEFT JOIN l C ON L.id = C.id AND L.SERVICE_DATE = C.SERVICE_DATE 
       AND C.FIRST_DT IS NOT NULL 
       AND C.FIRST_A_DT IS NOT NULL AND L.IS ='1' 
       AND L.FIRST_DT IS NOT NULL 
GROUP BY l.program, l.id 

這where條件是不必要的，除非你正在從某種過濾器的動態級聯這個SQL。