You can use any number of different aggregate functions in a query. They will all be calculated for each group. Consider the following example --
myTable ID Category Rating 1 foo 7 2 bar 2 3 foo 5 4 foo 3 5 bar - 6 bar 4
If you run the following query --
select Category , count(*) as Records , count(Rating) as Ratings , sum(Rating) as S , max(Rating) as M , avg(Rating) as A from myTable group by Category
... then you will get these results --
Category Records Ratings S M A foo 3 3 15 7 5 bar 3 2 6 4 3
Notice that all aggregate functions except COUNT(*) ignore the NULL Rating for the ID=5 row. COUNT(*) counts rows, whereas COUNT(col) counts non-null values.
So to answer your question, just go ahead and use SUM() and MAX() in the same query.
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This was first published in April 2002