In other word, aggregation is combining multiple pieces of data into one unit.
The container entity may be:
- another entity (for instance, a class that has student)
- or a nominal attribute (a tag, label or class) (for instance, a color that is present on sweaters)
aggregation is also known as:
- whole/part relationship
The container is said to be an aggregate/compound type.
- Library and Students. Because the student can exist without library, the relation between student and library is aggregation.
- professor and students: when the professor dies the students do not die along with him or her.
A aggregation is: