Tag: referential integrity constraint

Constraints in DBMS | Types of Constraints in DBMS

Constraints in DBMS-

 

  • Relational constraints are the restrictions imposed on the database contents and operations.
  • They ensure the correctness of data in the database.

 

Types of Constraints in DBMS-

 

In DBMS, there are following 5 different types of relational constraints-

 

 

  1. Domain constraint
  2. Tuple Uniqueness constraint
  3. Key constraint
  4. Entity Integrity constraint
  5. Referential Integrity constraint

 

1. Domain Constraint-

 

  • Domain constraint defines the domain or set of values for an attribute.
  • It specifies that the value taken by the attribute must be the atomic value from its domain.

 

Example-

 

Consider the following Student table-

 

STU_IDNameAge
S001Akshay20
S002Abhishek21
S003Shashank20
S004RahulA

 

Here, value ‘A’ is not allowed since only integer values can be taken by the age attribute.

 

2. Tuple Uniqueness Constraint-

 

Tuple Uniqueness constraint specifies that all the tuples must be necessarily unique in any relation.

 

Example-01:

 

Consider the following Student table-

 

STU_IDNameAge
S001Akshay20
S002Abhishek21
S003Shashank20
S004Rahul20

 

This relation satisfies the tuple uniqueness constraint since here all the tuples are unique.

 

Example-02:

 

Consider the following Student table-

 

STU_IDNameAge
S001Akshay20
S001Akshay20
S003Shashank20
S004Rahul20

 

This relation does not satisfy the tuple uniqueness constraint since here all the tuples are not unique.

 

3. Key Constraint-

 

Key constraint specifies that in any relation-

  • All the values of primary key must be unique.
  • The value of primary key must not be null.

 

Example-

 

Consider the following Student table-

 

STU_IDNameAge
S001Akshay20
S001Abhishek21
S003Shashank20
S004Rahul20

 

This relation does not satisfy the key constraint as here all the values of primary key are not unique.

 

4. Entity Integrity Constraint-

 

  • Entity integrity constraint specifies that no attribute of primary key must contain a null value in any relation.
  • This is because the presence of null value in the primary key violates the uniqueness property.

 

Example-

 

Consider the following Student table-

 

STU_IDNameAge
S001Akshay20
S002Abhishek21
S003Shashank20
Rahul20

 

This relation does not satisfy the entity integrity constraint as here the primary key contains a NULL value.

 

5. Referential Integrity Constraint-

 

  • This constraint is enforced when a foreign key references the primary key of a relation.
  • It specifies that all the values taken by the foreign key must either be available in the relation of the primary key or be null.

 

Read more- Foreign Key in DBMS

 

Important Results-

 

The following two important results emerges out due to referential integrity constraint-

  • We can not insert a record into a referencing relation if the corresponding record does not exist in the referenced relation.
  • We can not delete or update a record of the referenced relation if the corresponding record exists in the referencing relation.

 

Example-

 

Consider the following two relations- ‘Student’ and ‘Department’.

Here, relation ‘Student’ references the relation ‘Department’.

 

 

Student

STU_IDNameDept_no
S001AkshayD10
S002AbhishekD10
S003ShashankD11
 S004RahulD14 

 

Department

Dept_noDept_name
D10ASET
D11ALS
D12ASFL
D13ASHS

 

Here,

  • The relation ‘Student’ does not satisfy the referential integrity constraint.
  • This is because in relation ‘Department’, no value of primary key specifies department no. 14.
  • Thus, referential integrity constraint is violated.

 

Handling Violation of Referential Integrity Constraint-

 

To ensure the correctness of the database, it is important to handle the violation of referential integrity constraint properly.

 

Next Article- Handling Violation of Referential Integrity Constraint

 

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Referential Integrity Constraint | Violation

Referential Integrity Constraint-

 

Before you go through this article, make sure that you have gone through the previous article on Constraints in DBMS.

 

We have discussed-

  • Relational constraints impose the restrictions on the database to ensure the correctness of data.
  • There are following 5 different types of relational constraints-

 

 

  • Referential Integrity constraint is enforced when a foreign key references the primary key of a relation.
  • It specifies that all the values taken by the foreign key must either be available in the relation of the primary key or be null.

 

Also read- Foreign Key in DBMS

 

Referential Integrity Constraint Violation-

 

There are following three possible causes of violation of referential integrity constraint-

 

Cause-01: Insertion in a referencing relation

Cause-02: Deletion from a referenced relation

Cause-03: Updation in a referenced relation

 

Cause-01: Insertion in a Referencing Relation-

 

  • It is allowed to insert only those values in the referencing attribute which are already present in the value of the referenced attribute.
  • Inserting a value in the referencing attribute which is not present in the value of the referenced attribute violates the referential integrity constraint.

 

Example-

 

Consider the following two schemas-

 

 

Here, relation “Student” references the relation “Branch”.

 

Student

Roll_noNameAgeBranch_Code
1Rahul22CS
2Anjali21CS
3Teena20IT

 

Branch

Branch_CodeBranch_Name
CSComputer Science
EEElectronics Engineering
ITInformation Technology
CECivil Engineering

 

Here,

  • In relation “Student”, we can not insert any student having branch code ME (Mechanical Engineering).
  • This is because branch code ME is not present in the relation “Branch”.

 

Cause-02: Deletion from a Referenced Relation-

 

  • It is not allowed to delete a row from the referenced relation if the referencing attribute uses the value of the referenced attribute of that row.
  • Such a deletion violates the referential integrity constraint.

 

Example-

 

Consider the above two relations,

  • We can not delete a tuple from the relation “Branch” having branch code ‘CS’.
  • This is because the referencing attribute “Branch_Code” of the referencing relation “Student” references the value ‘CS’.
  • However, we can safely delete a tuple from the relation “Branch” having branch code ‘CE’.
  • This is because the referencing attribute “Branch_Code” of the referencing relation “Student” does not uses this value.

 

Handling the Violation-

 

The violation caused due to a deletion from the referenced relation can be handled in the following three ways-

 

Method-01:

 

  • This method involves simultaneously deleting those tuples from the referencing relation where the referencing attribute uses the value of referenced attribute being deleted.
  • This method of handling the violation is called as On Delete Cascade.

 

Method-02:

 

  • This method involves aborting or deleting the request for a deletion from the referenced relation if the value is used by the referencing relation.

 

Method-03:

 

  • This method involves setting the value being deleted from the referenced relation to NULL or some other value in the referencing relation if the referencing attribute uses that value.

 

Cause-03: Updation in a Referenced Relation-

 

  • It is not allowed to update a row of the referenced relation if the referencing attribute uses the value of the referenced attribute of that row.
  • Such an updation violates the referential integrity constraint.

 

Example-

 

Consider the above relation.

  • We can not update a tuple in the relation “Branch” having branch code ‘CS’ to the branch code ‘CSE’.
  • This is because referencing attribute “Branch_Code” of the referencing relation “Student” references the value ‘CS’.

 

Handling the Violation-

 

The violation caused due to an updation in the referenced relation can be handled in the following three ways-

 

Method-01:

 

  • This method involves simultaneously updating those tuples of the referencing relation where the referencing attribute uses the referenced attribute value being updated.
  • This method of handling the violation is called as On Update Cascade.

 

Method-02:

 

  • This method involves aborting or deleting the request for an updation of the referenced relation if the value is used by the referencing relation.

 

Method-03:

 

  • This method involves setting the value being updated in the referenced relation to NULL or some other value in the referencing relation if the referencing attribute uses that value.

 

Next Article- Closure of an Attribute Set

 

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