query problem

[spec]

Hi friends,

I had two tables parent(f1) , child(f1,f2,f3).
I had to display f3 value for parent(f1) when f1 has only one f2,
if f2 values are more than one for single f1 and if all f3 values are 0 then
i should display f3 as 0, else if some values for f3 are nonZeros and some are Zeros then
is should display f3 value as '-'.

How to do this?
please any suggestions?

Thanks.