postgresql中组内操作之间的时间间隔

Question

I have a table with users, risk level, and timestamp of that risk. It looks like this:

user	risk	timestamp
Jim	high	2022-01-01
Jim	low	2022-01-02
Bob	low	2022-01-03
Bob	high	2022-01-05
Bob	low	2022-01-07
Kev	high	2022-01-08
Kev	low	2022-01-10
Kev	high	2022-01-15
Kev	low	2022-01-23

I want to find how long it takes for a user to go from high risk to low , so it would look something like this:

user	high_timestamp	low_timestamp	duration
Jim	2022-01-01	2022-01-02	1 day
Bob	2022-01-05	2022-01-07	2 day
Kev	2022-01-08	2022-01-10	2 day
Kev	2022-01-15	2022-01-23	8 day

Answer 1

You could accomplish this with window functions and FILTER , but the way with the least code would be to do something like:

SELECT
    "user",
    timestamp AS high_timestamp,
    (SELECT timestamp AS prior_high_timestamp
     FROM t highs
     WHERE highs."user"=t.user AND highs.event='high' AND highs.time<t.time
     ORDER BY time DESC LIMIT 1)
FROM t
WHERE risk='low';

If performance is a concern I might test different options, however.

Answer 2

You can use LEFT JOIN LATERAL to get user rows with high value in risk column not followed by low value at all. It would be like:

SELECT 
   high."user",
   high.timestamp as high_timestamp, 
   low.timestamp as low_timestamp,
   CAST(low.timestamp - high.timestamp AS TEXT) || ' day' as duration
FROM user_risk high
LEFT JOIN LATERAL (
    SELECT
        "user", 
        timestamp
    FROM user_risk 
    WHERE risk = 'low' AND "user" = high."user" AND timestamp >= high.timestamp
    ORDER BY timestamp
    LIMIT 1
) low ON TRUE
WHERE risk = 'high'

Please, check working demo at https://www.db-fiddle.com/f/gRhrkRrvLfBvYGXFAVU7Vc/1 .

I've added one more row at the end ('Kev', 'high', '2022-01-25') , that is not followed by low risk value to demonstrate the case