Difference Between Primary key and Unique key

Key Difference between Primary Key and Unique Key

There can be one primary key in a table, while there can be multiple unique keys in the table.
The purpose of the primary key is to enforce entity integrity; on the other hand, the purpose of the unique key is to enforce unique data.
In primary keys, the default index is clustered, whereas in unique keys, the default index is not-clustered
The Primary key does not allow null columns, whereas the unique key allows null columns.
In the primary key, duplicate keys are not allowed, while in a unique key, if one or more key parts are null, then duplicate keys are allowed.

Here, I have analyzed the difference between the primary key and the unique key and will comprehensively evaluate their pros and cons.

What is Primary Key?

A primary key constraint is a column or group of columns in a table that uniquely identifies every row in that table. The Primary key can’t be a duplicate, meaning the same value can’t appear more than once in the table.

A table can have only one primary key. Primary key can be defined at the column or table level. If you create a composite primary key, it should be defined at the table level.

Example of Creating a Primary Key

The following example describes that there is a table called student. It contains five attributes: 1) StudID, 2) Roll No., 3) First Name, 4) Last Name, and 5) Email.

The Roll No attribute can never contain a duplicate or null value. This is because every student enrolled in a university can have a unique roll number. You can easily identify each row of a table with the student’s roll number. So, it is considered a primary key.

Example of Creating Primary Key — Primary Key example

Features of Primary Key

Based on my observations, here are the important features of a primary key:

The primary key implements the entity integrity of the table.
You can keep only one primary in the table.
The primary key contains one or more table columns.
Columns are defined as not null.

Why use Primary Key?

Reflecting on my practical knowledge, here are the important reasons to use a primary key:

The main aim of the primary key is to identify each and every record in the database table.
You can use a primary key when you do not allow someone to enter null values.
If you delete or update a record, the action you specified will be undertaken to ensure database data integrity.
Perform a restrict operation to reject the delete or update operation for the parent table.
Data are organized in the sequence of clustered indexes whenever you physically organize a DBMS table.

What is Unique Key?

A unique key is a group of one or more fields or columns in a table that uniquely identify a database record.

A unique key is the same as a primary key, but it can accept one null value for a table column. It also cannot contain identical values. Unique constraints are referenced by the foreign key of other tables.

Example of Creating Unique Key

Consider the same student table with the following attributes: 1) StudID, 2) Roll No., 3) First Name, 4) Last Name, and 5) Email.

Stud ID can have a unique constraint where entries in the Stud ID column can be unique because each student at a university must have a unique ID number. If a student is changing universities, he or she would not have any student ID. The entry may have a null value, as only one null is allowed in the unique key constraint.

Example of Creating Unique Key — Unique key example

Features of Unique key

Here, I will outline the important features of a unique key:

You can define more than one unique key in the table.
By default, unique keys are in non-clustered unique indexes.
It consists of one or more table columns.
The table column can be null, but only one null per column is preferable.
A unique constraint can be easily referenced by a foreign key constraint.

Why use Unique Key?

Here are the important reasons I recommend using a unique key:

The purpose of a unique key is to make sure that the information in the column for each table record is unique.
When you allow the user to enter the null value.
Unique key is used because it creates a non-clustered index by default.
Unique key can be used when you have to keep null values in column.
When one or more than one field/columns of a table uniquely identify a record in a database table.

Differences Between Primary Key and Unique Key

In my analysis, here are the main differences between a primary key and a unique key:

Comparison Basis	Primary Key	Unique Key
Number of Key	There can be one primary key in a table	There can be multiple unique keys in the table
NULL	It does not allow null columns.	It allows null columns.
Index	Default index is clustered	Default Index is no-clustered
Purpose	The purpose of the primary key is to enforce entity integrity.	The purpose of a unique key is to enforce unique data.
Use	It is a SQL constraint that allows you to uniquely identify each record or row in the database table.	It is an SQL constraint that does not allow the same value to be assigned to two isolated records in a database table.
Value Modification	In the primary key, duplicate keys are not allowed.	In a unique key, if one or more key parts are null, then duplicate keys are allowed.
Syntax	Primary key can be created using syntax: CREATE TABLE Employee ( ID int PRIMARY KEY, Name varchar(255), City varchar(150) )	Unique key can be created using syntax: CREATE TABLE Employee ( ID int UNIQUE. Name varchar(255) NOT NULL. City varchar(150) )

You can also explore: Difference Between Primary Key and Foreign Key

How to Choose Between a Primary Key and a Unique Key

Unique keys are better when we have columns that we know shouldn’t contain duplication.
The primary key is ideal when you cannot keep null in the table. It can also be used when you have a foreign key in another table to create a relationship.

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