stumped with sql

[exc]

i have one table that keeps the ending balance on a customer's account.

i have another table that keeps the ledger for all events.

if the ending balance on the account is negative (ie we owe the customer money i want to pull the account. that is easy.

but i want to filter the selection even more by getting only those accounts that have payments made but no debits to the account. that is...i want to know if the customer's are paying on accounts that have nothing assessed to them.

i am at a loss as to how to do this in cr using sql...can it be done?