I wonder if the 'debits and credits' example is still realistic. Is it not that banks record all individual money transfers and keep them on record for years (often due to national laws that require this). Then the balance on a certain account is a simply a selection/view on the set of all transactions. For efficiency reasons you may want to store the result in some table. If there is a limit on withdrawals on a certain account, you do need to calculate the balance first and then a transaction mechanism is needed to assure that there are not several money transfer that individually do not surpass the limit but together do.