Simplify sql query(date to period) - sql

Question

got this table:
initial table
Need to get table like this:
Target table
what I used:
Select ID_CLIENT, BALANCE, HIST_DT as Start_dt, isnull(lead(hist_dt) over (partition by id_client order by hist_dt asc), '2999.12.31') as End_dt
from (
select ID_CLIENT, ID_STATUS, balance, hist_dt, lag(id_status) over (partition by id_client order by id_status) as Prev_ID_status
from Client_History) a
where a.ID_STATUS = a.Prev_ID_status or a.ID_STATUS = 1
order by ID_CLIENT, HIST_DT
I think its very complicated. Will be glad to hear any suggestions of simplyfing this query.

Answer 1

This is a gaps-and-islands problem, most easily solved with the difference of row numbers and aggregation:
select id_client, balance, min(hist_dt), max(hist_dt)
from (select ch.*,
row_number() over (partition by id_client order by balance_hist_dt) as seqnum,
row_number() over (partition by id_client, balance order by balance_hist_dt) as seqnum_2
from client_history ch
) ch
group by id_client, balance, (seqnum - seqnum_2)
order by id_client, min(hist_dt);
Why this works is a little tricky to describe. But if you look at the results of the subquery, you will see how the difference captures adjacent rows with the same balance.