select half of partition by for a sum() - sql

Question

I have a query that is calculating the sum of a partition by and giving me a running total by a category.
this part works well, now, I would like the sum of only the top 50% of the partition by.
maybe a table example will show:
╔═══════╦══════════════════════════╦════════════════════════════╗
║ col_1 ║ sum of partition by ║ sum of 50% of partition by ║
╠═══════╬══════════════════════════╬════════════════════════════╣
║ 1 ║ 36 (this is 1+2+3+...8) ║ 10 (1+2+3+4) ║
╠═══════╬══════════════════════════╬════════════════════════════╣
║ 2 ║ 35 (this is 2+3+4+....8) ║ 9 (2+3+4) ║
╠═══════╬══════════════════════════╬════════════════════════════╣
║ 3 ║ 34 ║ 7 (3+4) ║
╠═══════╬══════════════════════════╬════════════════════════════╣
║ 4 ║ 33 ║ 4 ║
╠═══════╬══════════════════════════╬════════════════════════════╣
║ 5 ║ 32 ║ null ║
╠═══════╬══════════════════════════╬════════════════════════════╣
║ 6 ║ 31 ║ null ║
╠═══════╬══════════════════════════╬════════════════════════════╣
║ 7 ║ 30 ║ null ║
╠═══════╬══════════════════════════╬════════════════════════════╣
║ 8 ║ 29 ║ null ║
╚═══════╩══════════════════════════╩════════════════════════════╝
right now I'm doing
sum(col_) over(partition by <another col> order by <a third col>) as [sum of partition by ]
then I later need to add another column for this calculation over the 25% so you get the idea.

Answer 1

You can use conditional logic by enumerating the rows and filtering. The following uses standard SQL syntax:
select x,
sum(x) over (order by x desc),
sum(x) filter (where seqnum <= 0.5 * cnt) over (order by x desc),
sum(x) filter (where seqnum <= 0.25 * cnt) over (order by x desc)
from (select x, count(*) over () as cnt,
row_number() over (order by x) as seqnum
from generate_series(1, 8, 1) gs(x)
) x
order by x;
Here is a db<>fiddle.
Although standard, Postgres is the only database that supports filter. The logic can easily be replaced with sum(case . . .).
Here is a db<>fiddle using SQL Server instead. The corresponding code is:
with gs as (
select 1 as x union all
select x + 1 from gs where x < 8
)
select x,
sum(x) over (order by x desc),
sum(case when seqnum <= 0.5 * cnt then x end) over (order by x desc),
sum(case when seqnum <= 0.25 * cnt then x end) over (order by x desc)
from (select x, count(*) over () as cnt,
row_number() over (order by x) as seqnum
from gs
) x
order by x;