SQL joining, summing from sub queries to calculate a total aggregate amount - sql

Question

I've done all of this in active record, but it's not going to work with pagination so I'd like to switch to a SQL query with OFFSET so I can query efficiently. - Rather than getting User.all, then working out the calculations on related objects and then compiling it all into a bundled array and finally sending it up to the view I'd like to handle the calcs in a find_by_sql command so it's easier to manage pagination etc.
Trying to work out a users total amounts invested + their residual uninvented amounts in my little stock market simulator I'm playing with.
Have a share prices table that has multiple entries for each new share price for each company, so want to:
a) select the last entry from that table to get the latest share
price
b) ownerships shows what users from users table own what
company shares
c) So if we multiply shares_owned from the ownerships
table with the latest share price from a) then we get the total
amount invested
d) Once we have the total amount invested across all
companies, we need to add on what uninvested dollars the user has
associated with them so total invested + u.dollars should give us the
total valuation for a given user.
e) what screws me up is that a user might not own anything at a particular time, which means he will have no entries in the ownership table. In that case the query needs to only return his uninvested u.dollars amount.
Trying to get that total valuation per customer and order by 'richest' user:
select u.id,
sum(
((
select s.price from shareprices s
WHERE s.company_id = o.company_id and u.id = o.user_id
ORDER BY id DESC LIMIT 1) * o.shares_owned
))
+ u.dollars as totalAmount
from users as u full outer join ownerships as o on u.id = o.user_id group by u.id order by totalamount ASC
That returns fine for where there are ownerships and the calculations for total invested work out, but for users who only have uninvested dollars how can i get them to show in that summed column, so essentially its 0 (ie. no owned investment amounts because they own shares in no companies) + u.dollars to get how much they have, but I don't understand how to make SQL do that.
I am hoping to avoid needing a pgsql function() to achieve this logic but if the above didn't make it obvious, I'm terrible at SQL and only learning it now. I hope my explanation is even understandable!

Answer 1

You can add a colaesce around the part of the calculation that needs to treat nulls as zeros (its not clear to me which part needs to )
sum(COALESCE
((
select s.price from shareprices s
WHERE s.company_id = o.company_id and u.id = o.user_id
ORDER BY id DESC LIMIT 1) * o.shares_owned
)) , 0))
+ u.dollars totalAmount