Dealing with constraints violation, Database Management System

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If the deletion violates referential integrity constraint, then three alternatives are available:

Default option: - refuse the deletion. It is the job of the DBMS to describe to the user why the deletion was rejected.
Attempt to propagate (or cascade) the deletion by deleting tuples that reference the tuple that is being deleted.
Vary the value of referencing attribute that origins the violation.

Example:

Let R:

A#	B	C^
A1	B1	C1
A2	B3	C3
A3	B4	C3
A4	B1	C5

C#	D
C1	D1
C3	D2
C5	D3

Note:

1) '#' shows the Primary key of a relation.

2) '^' shows the Foreign key of a relation.

(1) Delete a tuple with C# = 'C1' in Q.

Violated constraint: - Referential Integrity constraint

Motive: - Tuples in relation A refer to tuple in Q.

Dealing: - Options available are

1) Decline the deletion.

2) DBMS may automatically delete all tuples from relation S and Q with C

# = 'C1'. This is known as cascade detection.

3) The one more option would result in putting NULL value in R where C1 exist, which is the initial tuple R in the attribute C.

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