The expression of object-orientation can be done in one of two ways.

1. The concepts of object-orientation can be used purely as a design tool, and are encoded into, e.g., a relational database. E.g., ER modeling.

2. The concepts of object-orientation are incorporated into a language that is used to manipulate the database.

There are several possible languages into which the concepts can be integrated.

(a) Extend a DML such as SQL by adding complex types and object-orientation. Systems that provide object-oriented extensions to relational systems are called object-relational systems.

(b) Take an existing object-oriented programming language and extend it to deal with database. Such languages are called persistent programming languages.