Suppose I have a table Tab1 with attributes - a1, a2, ... etc. None of the attributes are unique.
What will be the nature of the following query? Will it return a single row always?
SELECT a1, a2, sum(a3) FROM Tab1 GROUP BY a1, a2
GROUP BY returns a single row for each unique combination of the GROUP BY fields. So in your example, every distinct combination of (a1, a2) occurring in rows of Tab1 results in a row in the query representing the group of rows with the given combination of group by field values . Aggregate functions like SUM() are computed over the members of each group.