3 level archictecture

logical data independence: Adding or removing new entities, attributes, or realationships shouldn't mess with existing external schema's or make changing applications necessary.

physical data independence: Changing the internal schema should be possible with out changing the conceptual/external schemas.



Referential Integrity: If a foriegn key exists in a relation the value of teh foreign key must match the candidate dey in the home relation.

Entity Integrity: In a base relation no attribute of a primary key can be null.

Data Definition Language: Describes + names entities attributes and relationships plus associated integrity and security constraints.

Data Manipulation Language(DML): Provides a set of operations to support the basic data manipulation operations (inserting, modifying, retrieving, and deleting data) on the database.

Procedural DML: Lets the user state what data is needed and how to get it.

Non procedural DML: Lets the user state what data is needed, but not how to get it.