Aggregation is often found in queries given the frequency of requirements of finding an average, the maximum or how many times something happens.
Aggregation is an abstraction concept for building composite objects from their component objects.
There are three cases where this concept can be related to the EER model.
The first case is the situation in which we aggregate attribute values of an object to form the whole object.
The second case is when we represent an aggregation relationship as an ordinary relationship.
The third case, which the EER model does not provide for explicitly, involves the possibility of combining objects that are related by a particular relationship instance into a higher-level aggregate object.
The functions supported in SQL are average, minimum, maximum, count, and sum.
Aggregation can itself be of different types including aggregation that only requires one relation, for example finding the maximum mark in a subject, or it may involve a relation but require something like finding the number of students in each class.
The latter aggregation would obviously require some grouping of the tuples in the relation before aggregation can be applied.
The abstraction of association is used to associate objects from several independent classes