How do I iterate a table according to fixed time intervals in Postgres?

Question

I want to obtain all rows for which, every interval of 48h in a given period of time, are satisfied the following conditions. Every time all of them are true, I put a flag with

Answer 1

This is a stab at an answer. It really needs more complete data to be taken seriously. Here goes:

SELECT
    *, 1 AS flag
FROM
(SELECT
    *,
    valuenum - LAG(valuenum, 1) OVER(partition by item) AS diff,
    intime - LAG(intime, 1) OVER(partition by item) AS time_diff
FROM
    lab L
JOIN
    icu I
ON
    L.id_sub = I.id
WHERE
    L.item = 50912
AND
    L.charttime < I.intime AND L.charttime > (I.intime - INTERVAL '7 DAY')
) AS select_diff

WHERE
    select_diff.diff > 0.3
AND
    select_diff.time_diff <interval '48 hours';