Question

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

Answer 1

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.