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Define entity integrity and referential integrity. How does SQL allow specification of these?
Ans: Entity Integrity Rule- If the attribute A of relation R is a prime attribute of R after that A cannot accept null values.
Referential Integrity Rule- In referential integrity, it is made sure that a value that appears in one relation for a given set of attributes as well appears for a particular set of attributes in another relation.
In SQL, referential integrity and entity integrity rules are implemented as constraints on the relation known as reference Key constraint and primary Key constraint correspondingly.
These constraints can be fixed with relation at the time of creation of the relations or after the creation of the relations by altering the definition of the relations. For instance:
Foreign Keys and NULL Let us take the relation: DEPT DEPT ID DNAME BUDGET D1 M
Timestamps For Read operations: If TS (T) If TS (T) >= W-timestamp (Q), then the transaction T is trying to read a value of data item Q which has been written and committed
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