Newb help in designing query to subtract results of two queries in same table - sql

Question

I have seen other questions like this one but feel mine is a bit different, or didn't quite understand the SQL in the other questions...so my apologies if this one is redundant or very easy..
Anyway, I have an accounting transaction DB that stores every transaction posting within our financial system on one line. What I am trying to do is net the sum of the debits and the credits for each GL account.
Here are the two basic queries I am executing to get the results that I would like to net.
Query 1 gives me the sum of all debit transactions posting to each gl account:
Select gl_debit, sum (amt) from FISC_YEAR2014 where fund = 'XXX'
group by gl_debit
Query 2 gives me the sum of all credit transactions posting to each gl account:
select gl_credit, sum (amt) from FISC_YEAR2014 where fund = 'XXX'
group by gl_credt
Now I would to subtract the credit amounts from the debit amounts to get net totals for each gl account. Make sense?
Thanks.

Answer 1

There are two ways to do this depending our your table definition. I think your situation is the first.
This is the normal way assuming credits and debits are in separate columns:
SELECT sum(gl_debit)-sum(gl_credit) as net_debit
FROM FISC_YEAR2014
WHERE fund = 'XXX'
This is the other way assuming direction is indicated by a separate column:
SELECT SUM(IF(is_debit=1,amount,-1*amount)) as net_debit
FROM FISC_YEAR2014
WHERE fund = 'XXX'
See also:
MySQL 'IF' in 'SELECT' statement
Can't calculate totals in general ledger report
What's a good way to store a financial ledger?

Answer 2

I believe this is what you need:
select
gl_account,
sum(amt)
from
(
select gl_debit gl_account,
sum(-amt) amt
from fisc_year2014
where fund = 'XXX'
group by gl_debit
union all
select gl_credit,
sum(amt)
from fisc_year2014
where fund = 'XXX'
group by gl_credit
)
group by
gl_account
There are two SELECTs: one to get the (negative) debits and another to get the credits. They are UNIONed to create a two-column result. The outer SELECT then aggregates the total sum by the gl_account code. If there is a mismatch (a gl_debit without a gl_credit, or vice-versa), then its amount would still be displayed.
SQLFiddle here (I added another row to show the effect of mismatched IDs)

Answer 3

To do this you should SUM the debits and credits separately in subqueries, then join those subqueries on gl_credit = gl_debit.
SELECT COALESCE(gl_credit, gl_debit) AS Id
,COALESCE(d.amt,0)-COALESCE(c.amt,0) AS Net
FROM (
SELECT gl_debit, SUM(amt) AS amt
FROM FISC_YEAR2014
GROUP BY gl_debit
) d
FULL OUTER JOIN (
SELECT gl_credit, SUM(amt) AS amt
FROM FISC_YEAR2014
GROUP BY gl_credit
) c ON d.gl_debit = c.gl_credit
ORDER BY COALESCE(gl_credit, gl_debit)
SQLFiddle
Outputs:
ID Net
-----------
101 -475
201 225
301 500
501 -250
If I were you rather than using a FULL OUTER JOIN, I'd select the ids from the accounts table or wherever you store them, then LEFT JOIN both of the subqueries to it, you haven't shown any other tables though so I can only speculate.