postgresql - SQL窗口函数检测列值的变化

Question

我想检测这个(示例)数据库中列值的变化

    WITH events(id, row,event) AS (
    VALUES
    (1,1, 0 )
   ,(1,2, 0 ) 
   ,(1,3, 1 )
   ,(1,4, 0 )
   ,(1,5, 1 )
   ,(2,1, 0 )
   ,(2,2, 1 )
   ,(3,1, 0 )
   ,(3,2, 0 )
   )
   select * from events

我正在寻找的是一个新列“代码”的代码，它在之后切换到 1de event 列显示 1。在同一 id 中，代码保持为 1。对于这个例子，这个新列看起来像这样

    WITH events2(id, row,event, code) AS (
    VALUES
    (1,1, 0, 0 )
   ,(1,2, 0, 0 ) 
   ,(1,3, 1, 0 )
   ,(1,4, 0, 1 ) -- notice the switch here
   ,(1,5, 1, 1 ) -- 
   ,(2,1, 0, 0 )
   ,(2,2, 1, 0 )
   ,(3,1, 0, 0 )
   ,(3,2, 0, 0 )
   )
   select * from events2

我有预感答案将与这个问题的答案相关:PostgreSQL window function: partition by comparison

不知何故，我无法自己解决这个问题..

彼得

Answer 1

在标量子查询上合并:

WITH events(id, zrow, zevent) AS (
    VALUES
    (1,1, 0 ) ,(1,2, 0 ) ,(1,3, 1 ) ,(1,4, 0 ) ,(1,5, 1 )
   ,(2,1, 0 ) ,(2,2, 1 )
   ,(3,1, 0 ) ,(3,2, 0 )
   )
SELECT id, zrow, zevent
        , COALESCE((SELECT 1 FROM events ex WHERE ex.id = ev.id AND ex.zrow < ev.zrow AND ex.zevent> 0),0) AS oevent
 FROM events ev
        ;

---

或者，通过将 bool 值 EXISTS() 类型转换为 INTEGER 来避免 COALESCE():

WITH events(id, zrow,event) AS (
    VALUES
    (1,1, 0 ) ,(1,2, 0 ) ,(1,3, 1 ) ,(1,4, 0 ) ,(1,5, 1 )
   ,(2,1, 0 ) ,(2,2, 1 )
   ,(3,1, 0 ) ,(3,2, 0 )
   )
SELECT id, zrow, event
        , EXISTS(SELECT 1 FROM events ex WHERE ex.id = ev.id AND ex.zrow < ev.zrow AND ex.event> 0)::integer AS oevent
FROM events ev
        ;

---

查找同一组内先前记录的 MAX() 值 (frame):

WITH events(id, zrow,event) AS (
    VALUES
    (1,1, 0 ) ,(1,2, 0 ) ,(1,3, 1 ) ,(1,4, 0 ) ,(1,5, 1 )
   ,(2,1, 0 ) ,(2,2, 1 )
   ,(3,1, 0 ) ,(3,2, 0 )
   )
, drag AS (
        SELECT id, zrow, event, MAX(event)
                OVER (PARTITION BY id
                        ORDER BY zrow
                        ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING
                        ) AS lagged
        FROM events ev
        )
SELECT id, zrow, event
        , COALESCE(lagged,0) AS oevent
        FROM drag dr
        ;

---

同样没有额外的 CTE:

WITH events(id, zrow,event) AS (
    VALUES
    (1,1, 0 ) ,(1,2, 0 ) ,(1,3, 1 ) ,(1,4, 0 ) ,(1,5, 1 )
   ,(2,1, 0 ) ,(2,2, 1 )
   ,(3,1, 0 ) ,(3,2, 0 )
   )
SELECT id, zrow, event, COALESCE(MAX(event) OVER (PARTITION BY id
                                                ORDER BY zrow
                                                ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING
                                ),0) AS lagged
        FROM events ev
        ;

---

执行自连接的另一种方法是使用递归查询。