In the relational data model, entity integrity is one of the three inherent integrity rules. Entity integrity is an integrity rule which states that every table must have a primary key and that the column or columns chosen to be the primary key should be unique and not null.
Within relational databases using SQL, entity integrity is enforced by adding a primary key clause to a schema definition. The system enforces Entity Integrity by not allowing operation (INSERT, UPDATE) to produce an invalid primary key. Any operation that is likely to create a duplicate primary key or one containing nulls is rejected. The Entity Integrity ensures that the data that you store remains in the proper format as well as comprehensible.
- Beynon-Davies P. (2004). Database Systems 3rd Edition. Palgrave, Basingstoke, UK. ISBN 1-4039-1601-2
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