Dealing with constraints violation, Database Management System

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

Q

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.

Posted Date: 3/6/2013 5:39:55 AM | Location : United States







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