1. We have assumed up to now that the relations we are given are the actual relations stored in the database.

2. For security and convenience reasons, we may wish to create a personalized collection of relations for a user.

3. We use the term view to refer to any relation, not part of the conceptual model, that is made visible to the user as a "virtual relation".

4. As relations may be modified by deletions, insertions and updates, it is generally not possible to store views.(Why?) Views must then be recomputed for each query referring to them.