Aggregation
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
.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
andHAVING
conditions is theHAVING
applies to all the groups generated from theGROUP BY
clause. While theWHERE
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>;
Last modified 3yr ago