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Entity Integrity

Entity integrity is concerned with ensuring that each row of a table has a unique and non-null primary key value; this is the same as saying that each row in a table represents a single instance of the entity type modelled by the table. A requirement of E F Codd in his seminal paper is that a primary key of an entity, or any part of it, can never take a null value.

Oracle, and most other relational database management systems, will enforce this.

Next: Column Constraints