Да, можете да направите случай:
select case when noOfGirls=0 then noOfBoys
else round(noOfBoys/noOfGirls) end as ration
from student;
Но вероятно искате:
select case when noOfGirls=0 then 1
else round(noOfBoys/noOfGirls) end as ration
from student;