Which is most correct - coalesce(sum(value)) OR sum(coalesce(value))?

Question

I have seen both on SO, but I wonder if there is a "best practice" or advantage to coalesce the sum before or after:

coalesce(sum(value),0)  OR  sum(coalesce(value),0)

Just curious...

Answer 1

They don't do the same thing. Putting the COALESCE inside does not do anything if you have no records. Putting it outside changes the result in that case too to 0 . Which to use depends on the result you want.

Answer 2

The SQL engine will probably optimize this anyway but using

coalesce(sum(value),0) 

may be a bit faster because the summing can be done without the need to process a function and at the end coalesce is called one time .

The secone version calls coalesce for every record and sets the value to 0 if there are null values in the summing.