Aggregation in DBMS

• 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