Select previous different value PostgreSQL - postgresql

Question

I have a table:
id
date
value
1
2022-01-01
1
1
2022-01-02
1
1
2022-01-03
2
1
2022-01-04
2
1
2022-01-05
3
1
2022-01-06
3
I want to detect changing of value column by date:
id
date
value
diff
1
2022-01-01
1
null
1
2022-01-02
1
null
1
2022-01-03
2
1
1
2022-01-04
2
1
1
2022-01-05
3
2
1
2022-01-06
3
2
I tried a window function lag(), but all I got:
id
date
value
diff
1
2022-01-01
1
null
1
2022-01-02
1
1
1
2022-01-03
2
1
1
2022-01-04
2
2
1
2022-01-05
3
2
1
2022-01-06
3
3

Answer 1

I am pretty sure you have to do a gaps-and-islands to "group" your changes.
There may be a more concise way to get the result you want, but this is how I would solve this:
with changes as ( -- mark the changes and lag values
select id, date, value,
coalesce((value != lag(value) over w)::int, 1) as changed_flag,
lag(value) over w as last_value
from a_table
window w as (partition by id order by date)
), groupnums as ( -- number the groups, carrying the lag values forward
select id, date, value,
sum(changed_flag) over (partition by id order by date) as group_num,
last_value
from changes
window w as (partition by id order by date)
) -- final query that uses group numbering to return the correct lag value
select id, date, value,
first_value(last_value) over (partition by id, group_num
order by date) as diff
from groupnums;
db<>fiddle here