Key features of Object Relational Data Model (E.g. SQL:2003 object model; LOBs, UDTs, OID, Multisets)
Introduction to UML Class
UML Class Notation
UML Association vs Composition vs Aggregation
UML Aggregation Example
UML Composition Example
Unidirectional Association: An object might store another object in a field. For example, people own books, which might be modeled by an owns field in Person objects.
However, a book might be owned by a very large number of people, so the reverse direction might not be modeled. The *'s in the figure indicate that a book might be owned any number of people, and that a person can own any number of books.
Bidirectional Association: Two objects might store each other in fields. For example, in addition to a Person object listing all the books that the person owns, a Book object might list all the people that own it.