Key Constraints and NULL Value Constraints

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In this class, we will understand Key Constraints and NULL Value Constraints.

We have already discussed the concepts of domain constraints.

Table of Contents

To understand this, let us consider the simple table given below.

Why should we have the ‘key’ constraint?

If we consider an employee table, every employee is unique.

We should repeat the details of the employee.

We have to identify an attribute for a table that should have all unique values for that unique identification.

The definition of a relation states that a Relation is defined as a Set of Tuples.

The definition of a set from our primary math states that all elements of the set are distinct.

For example, the set of natural numbers will not have repetitions.

{1, 2, 3, 4, 5, …..}.

To understand this, let us consider the simple table given below.

In the above table, the name of the employee should not be NULL.

There will not be an employee who does not have a name.

So we should not accept the details of the employee without a name on the table.