The company I'm working for allows their customers to have multiple accounts. It's also possible for an account to be in the negative. So, their customer balance table looks like this:
┌──────────────┬─────────┬─────────┐
│ Customer No. │ Account │ Balance │
├──────────────┼─────────┼─────────┤
│ 1 │ A │ 1.00 │
├──────────────┼─────────┼─────────┤
│ 1 │ B │ -2.00 │
├──────────────┼─────────┼─────────┤
│ 1 │ C │ 3.00 │
├──────────────┼─────────┼─────────┤
│ 2 │ A │ 4.00 │
├──────────────┼─────────┼─────────┤
│ 2 │ B │ -5.00 │
├──────────────┼─────────┼─────────┤
│ 2 │ C │ -6.00 │
├──────────────┼─────────┼─────────┤
│ 3 │ A │ 7.00 │
└──────────────┴─────────┴─────────┘
I'm trying to get a sum total of the balance column, excluding any customers that have a negative total balance. So, the result from the table above should be 9.00 -- Customers 1 and 3 have a positive total balance, so all their accounts are included, but customer 2 has a negative total balance, so none of their accounts are included.
(Note: This is a once-a-year query, so performance isn't really an issue.)