SQL subquery within a SELECT calculation - sql

Question

Very new to subqueries and find myself in need of help.
I would like to query from a single database. Within that query, I would like to calculate a variable from two variables with that database (SUBQ and TOTAL). My issue is this: my SUBQ variable needs to be subject to an additional set of WHERE constraints on top of those that will be employed for the whole query. Simplified example code below:
create table [blah]
as select date_part('YEAR',DATE) as Orig_Year,
sum([SUBQ variable])/sum(TOTAL) as UD_Rate
from [database]
where [full query requirements]
group by date_part('YEAR',DATE)
I have tried to create a subquery within that calculation by specifying a subquery in the FROM statement. So, for example,
select date_part('YEAR',DATE1) as Orig_year,
sum(a.SUBQ)/sum(b.TOTAL) as UD_Rate
from database b,
(select SUBQ
from database
where DATE2 is not null and
months_between(DATE3,DATE2) <= 100 and
VALUE1 in ('A','B')) a
where VALUE2 between 50.01 and 100
group by date_part('YEAR',DATE1)
Am I on the right track with my thinking here? I have yet to get anywhere close to a functional query and have had little luck finding a similar question online, so I'm at the point where I've tossed up my hands and come to you. Though I know little about them, would it be more appropriate to create a VIEW with the SUBQ value, and then merge it with the broader query?
Thoughts of pies and cakes for whoever is willing to assist me with this request. Thank you.

Answer 1

I think you just want condition aggregation in a window function. Something like this:
select sum(case when [subquery requirements] then t.subq else 0 end) / sum(t.Total)
from t;
I'm pretty sure this is what you are looking for. In terms of your create table:
select date_part('YEAR',DATE) as Orig_Year,
sum(case when ?? then Total else 0 end)/sum(TOTAL) as UD_Rate
from [database]
where [full query requirements]
group by date_part('YEAR', DATE);
I am guessing that the column to be compared is Total, subject to the conditions in the when.

Answer 2

Use Common-Table-Expression then:
-- Define the CTE expression name and column list.
WITH subquery (Orig_year, UD_Rate)
AS
-- Define the CTE query.
(
select date_part('YEAR',DATE1) as Orig_year,
sum(a.SUBQ)/sum(b.TOTAL) as UD_Rate
from database b,
(select SUBQ
from database
where DATE2 is not null and
months_between(DATE3,DATE2) <= 100 and
VALUE1 in ('A','B')) a
where VALUE2 between 50.01 and 100
group by date_part('YEAR',DATE1)
)
And then use subquery as you would use a table inside your main query