The aggregate, or aggregation functions, initially appear inside the
SELECTclause 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:
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
GROUP BYallows us to partition our relations into groups, and then compute aggregated aggregate functions over each group independently.
HAVINGcondition allows us to test filters on the results of aggregate values.
- The difference between the
HAVINGconditions is the
HAVINGapplies to all the groups generated from the
GROUP BYclause. While the
WHEREcondition applies to single rows at a time.
The syntax looks like this:
SELECT A1, A2, . . . , A(n)
FROM R1, R2, . . . , R(m)
GROUP BY <columns>