postgresql - sql窗口函数来检测列值的变化

我想在这个（示例）数据库中检测列值的变化

    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

我要找的是新列“code”的代码，该列在
de event列显示1。在同一个id内，代码保持1。
在本例中，这个新列将如下所示

    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
不知怎的，我自己都搞不懂。。
彼得

最佳答案

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boolean 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（帧）中以前记录的MAX（）值：

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。