In order to get round the divide by zero error, I generally do this:
PF1 AS QF1 / nullif((QF1 + QF2),0),
instead of
PF1 AS QF1 / (QF1 + QF2),
I.e. set the divisor to null, if the divisor is zero.
I used to do complicated Case statements, but then I needed to retype the divisor.
This way, I just add "NULLIF(" at the front and ",0)" at the end.
I'we written about it at Stack overflow
Best regards,
Henrik Staun Poulsen