mySQL select based on sum of rows compared to another field value

Question

I have a table of invoice items. A single transaction may result in multiple debit items and multiple credit items sharing the same invoice_set_id, I need to compare the sum of the debit items against the sum of the credit items and add the invoice_set_id to the result set if sum(debits) > sum(credits). It should also add to the result if there is no row with a credit amount. Using mySQL. Thanks for any help. Example table and result follows:

invoice_item_id  invoice_set_id  credit_debit  amount
62                a22             debit         15.00
63                a22             debit          8.00
64                a22             credit        23.00
65                b23             debit         44.00
66                c55             debit         15.00
67                c55             debit          2.00
67                c55             credit         8.00

Given the above, the result set should be:

invoice_set_id
b23
c55

Explanation: a22 is not returned because the debits and credits are equal, b23 is returned because it has a debit but no credit, and c55 is returned because the sum of the debits is greater than the single credit.

I appreciate any help with this. The actual query is more involved, but I think this particular problem is all I need help with.

Answer 1

You can do this simply using group by and having :

select invoice_set_id
from t
group by invoice_set_id
having sum(case when credit_debit = 'debit' then amount else 0 end) > sum(case when credit_debit = 'credit' then amount else 0 end) ;

Another way of expressing the having clause is:

having sum(case when credit_debit = 'debit' then amount
                when credit_debit = 'credit' then - amount
                else 0
           end) > 0;