How does SQL allow implementation of the entity integrity and referential integrity constraints

 Question: 4.3. How does SQL allow implementation of the entity integrity and referential integrity constraints described in Chapter 3? What about referential triggered actions? 

Solution: 

Entity integrity

Referential integrity

Entity integrity constraints

In the entity integrity constraint, it states that no primary key value can be NULL because. Primary key value is used to identify the individual tuples in a relation. And having NULL values for the primary key implies that we cannot identify some tuples.

Referential integrity constraints

It is specified between two relations and is used to maintain the consistency among tuples in the two relations and referential integrity constraints states that, a tuple in one relation that refers to another relation and must refer to an existing tuple in that relation.

Referential triggered actions

Schema designer can specify an alternative action to taken when a referential integrity constraint is violated by attaching a referential triggered action clause to any foreign key constraint.