Using Select Aggregate Statements

Hi Experts,

     I am getting mixed response regarding the use of aggregate statements in select query. My concerns are:

1) Using aggregate function in select query or processing in internal table is more efficient ?

2) If it is database dependent (as some may argue ), for which databases we have to use aggregate function and for which we should avoid to use it ?

3) Is there any relevant study regarding this topic,  I mean some statistics with the no of records, and the underlying database used.

Sam.