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Difference between Primary Key and Unique Key in SQL-Server

In this article I am going to explain Differnce between Primay Key and Unique Key in SQL-Server.

The PRIMARY Key and UNIQUE Key constraints both are similar and it provide unique enforce uniqueness of the column on which they are defined. Also these key’s can Uniquely identify each row in database table.

 

Difference between PRIMARY KEY and UNIQUE KEY below table lists:

  PRIMARY KEY UNIQUE KEY
Basic Difference
1

Primary key cannot have a NULL value. Because of this we refer:

PRIMARY KEY = UNIQUE KEY + Not Null CONSTRAINT.

Unique Constraint may have only one NULL value.
2 Each table can have only one primary key. Each table can have more than one Unique Constraint.
3 By default it adds a clustered index. But as per required we can create with none clustered index. By default it adds a UNIQUE non-clustered index. But as per required we can create with clustered index.
Advance Difference
4 Primary key can be related with another table's as a Foreign Key. Unique Constraint can not be related with another table's as a Foreign Key.
5
We can generated ID automatically with the help of Auto Increment field. Primary key supports Auto Increment value.
A primary key must be unique.
Unique Constraint doesn't supports Auto Increment value.
6 A primary key must be unique. A unique key does not have to be the primary key
7
We can't delete primary key value from the parent table which is used as a foreign key in child table. To delete we first need to 
Not applied
     


Thanks !

Brijesh Kumar

 

 

Thank You

About Author

Brijesh Kumar

Database Developer

I have more then 6 years Experience in Microsoft Technologies - SQL Server Database, ETL Azure Cloud - Azure SQL Database, CosmosDB, Azure Data Factory, PowerBI, Web Job, Azure Function, Azure Storage, Web Apps, Powershall and Database Migration On-Premise to Azure Cloud.
LinkedIn : https://www.linkedin.com



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