Conditional sum - Column values

Hey there! 

Does anyone know how to make a sum of column values by checking an other column values? What I need to do is show at the end of the table the sum of the credit value less debt values. I'm using "setSqlExp" with the SUM function. But that sum all without considering whether it is credit or debit.

Has anyone done something like that?

Thanks and sorry for my bad english.
