Select previous different value 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