sql window function to detect change in column values - postgresql

Question

I'd like to detect changes in column values in this (example) db
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
What I am looking for is code for a new column 'code' which switches to 1 AFTER
de event column shows a 1. Within the same id the code stays 1.
For this example this new column wil look like this
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
I have a hunch that the answer will be related to the answer on this question : PostgreSQL window function: partition by comparison
Somehow I cannot figure this out myself..
Peter

Answer 1

COALESCE over a scalar subquery:
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
;
Or, avoid the COALESCE() by typecasting the boolean EXISTS() to INTEGER:
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
;
Find the MAX() value over the previous records within the same group (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
;
The same without the extra 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
;
Another way to perform the self-join would be to use a recursive query.