线索和案例表达答案

【问题标题】：Lead and case expression线索和案例表达
【发布时间】：2019-02-06 17:39:20
【问题描述】：

我有这张桌子：

ID    Date
-----------------
1     1/1/2019
1     1/15/2019

预期输出：

ID   DATE      LEAD_DATE  
-------------------------
1    1/1/2019  1/14/2019
1    1/15/2019  SYSDATE

SQL：

SELECT 
    *,
    CASE 
        WHEN LEAD (a.date) OVER (PARTITION BY a.ID ORDER BY a.date) = TRUNC(a.date) THEN NULL
        ELSE LEAD (a.date) OVER (PARTITION BY a.id ORDER BY a.date) - NUMTODSINTERVAL(1,'second')
    END AS LEAD_DT
 FROM a

结果：

ID  DATE      LEAD_DATE
-------------------------
 1  1/1/2019    1/14/2019
 1  1/15/2019

case表达式中null时可以加上系统日期吗？

【问题讨论】：

标签： sql oracle lead case-expression

【解决方案1】：

使用NVL：

SELECT 
    a.*,
    NVL(CASE 
        WHEN LEAD (a.date) OVER (PARTITION BY H.ID ORDER BY a.date) = TRUNC(a.date) THEN NULL
        ELSE LEAD (a.date) OVER (PARTITION BY a.id ORDER BY a.date) - NUMTODSINTERVAL(1,'second')
    END, SYSDATE) AS LEAD_DT
FROM a

或者，更好：

SELECT 
    a.*,
    CASE LEAD (a.date) OVER (PARTITION BY a.ID ORDER BY a.date)
        WHEN TRUNC(a.date) THEN SYSDATE
        WHEN NULL THEN SYSDATE
        ELSE LEAD (a.date) OVER (PARTITION BY a.id ORDER BY a.date) - NUMTODSINTERVAL(1,'second')
    END AS LEAD_DT
 FROM a

【讨论】：

【解决方案2】：

使用COALESCE:。

SELECT a.*,
  CASE COALESCE(LEAD("Date") OVER (PARTITION BY ID ORDER BY "Date") - "Date", 0)
    WHEN 0 THEN SYSDATE
    ELSE LEAD("Date") OVER (PARTITION BY ID ORDER BY "Date") - INTERVAL '1' SECOND
  END AS LEAD_DT
FROM a

【讨论】：