Question

Explain the following violation with an illustration.

i)Domain constraint

ii)Key constraint

iii)Referential integrity

iv)entity integrity

Answer 1

¡) Domain constraint :-

-- Domain constraint simply defines the domain or set of values for an attribute.

--It says the value of the attribute must be available in the corresponding domain.

-- If the attribute datatype is an integer then attribute values must be in integer.

Let's take an example:

Student_id	Name	Age
101	Rahul	19
102	Ajay	C
B	Santosh	19

Here , value ‘B’ is not allowed since only integer values can be taken by the "Student_id" attribute.

And ,value 'C' is not allowed since only integer values can be taken by the "Age" attribute.

¡¡) Key Constraint:-

-- keys are simply an entity set which are used to identify an entity within its set uniquely.

-- Key Constraint says that all the values of primary key must be UNIQUE.

--And , the values of the primary key must not be a NULL value.

Let's take an example :

Student_id	Name	Age
101	Rahul	19
102	Ajay	18
101	Santosh	19

Here, Student_id is the primary key and this relation does not satisfy the key constraint as here all the values of primary key are not unique.

¡¡¡) Referential Integrity :-

-- Referential Integrity Constraint is specified between two tables.

-- 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.

Let's take an example :

Table -1:

Student_id	Name	Age	Phone_no
101	Rahul	19	1234
102	Ajay	18	5236
103	Santosh	19	9876

Here , Phone_no is a foreign key.

Table-1 relationships with Table-2 .

Table -2:

Phone_no	Address
1234	kurnool
5236	ongole

-- Here , Phone_no is the primary key.

-- Here , in Table-1 9876 is not allowed as Phone_no 9876 is not defined as a Primary key of Table-2 and in Table-1 Phone_no is a foreign key defined.

¡v) Entity Integrity :-

-- 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.

Let's take an example :

Student_id	Name	Age
101	Rahul	19
102	Ajay	18
	Santosh	19

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