Aggregation

Aggregation Functions

The aggregate, or aggregation functions, initially appear inside the SELECT clause of a query, and they perform computations over sets of values in multiple rows of our relations. The basic aggregation functions supported by every SQL systems are: MIN, MAX, SUM, AVG, and COUNT.

Two New Clauses

Now that the aggregation functions have been introduced, two new clauses can be added to the SQL select statements. These are the: GROUP BY, and the HAVING clauses.

• The GROUP BY allows us to partition our relations into groups, and then compute aggregated aggregate functions over each group independently.

• The HAVING condition allows us to test filters on the results of aggregate values.

• The difference between the WHERE and HAVING conditions is the HAVING applies to all the groups generated from the GROUP BY clause. While the WHERE condition applies to single rows at a time.

The syntax looks like this:

SELECT A1, A2, . . . , A(n)
FROM R1, R2, . . . , R(m)
WHERE <condition>
GROUP BY <columns>
HAVING <condition>;