Using cast inside sum or outside?

Transact-SQL
,
1

I work on sql server 2012 I face issue using cast first or sum ?
cast (sum(a) as decimal(18,2)
or
sum(cast (a as decimal(18,2)))
which is correct and more speed
a is nteger need to convert to decimal
so using cast inside sum or outside

2

It's more efficient to do the CAST after the SUM.

But, if the SUM would overflow the data type, then do the CAST before the SUM.

For example, you are adding a bunch of ints and total would exceed the max value for an int, then CAST to a bigint before doing the SUM.