which one to use count or sum funtion? and why?

[lakhansajwan]

i know sum is preffered over count but why????/

[sujeetsingh]

1,2,3 be values of field COST

then Count(COST)=3

while SUM(COST) =6

So this the difference ..and you can use it as required

[#3]

count is to know the number of values, so sum is a completely diffrent thing