Differences between primary key and unique key - SQL Server Interview Questions

Hi Friends,
In this in this session we are going to learn the differences between primary key and unique key
This is one of the most frequently asked sql server interview questions.
Before we discuss the difference between primary key and unique key, let's see what a primary key is and what a

unique key is.
Primary Key:
Primary key is a key which uniquely identifies each row/record in the table.
This implies if you have a primary key on a table, you can identify each and every row in that table uniquely

using primary key.
Unique Key:
Unique key constraint enforces the uniqueness on the columns in which it is defined.
That is Unique key will make sure only unique values are allowed in the columns in which it is defined.

Similarities:
Both Primary key and unique Key enforces uniqueness on the columns on which they are defined.
• Primary key and unique Key columns allow only unique values.
• Primary key and unique Key columns will avoid duplicates
• Both Primary key and unique Key columns uniquely identifies each row/record in the table

Differences:
PRIMARY KEY UNIQUE KEY
Primary Key Does not allow duplicates Unique key allows only one NULL
A table can have only 1 Primary Key A table can have multiple Unique Keys
By default Primary Key creates a Unique Clustered Index on the table By default Unique Key creates a Unique

Non-Clustered Index on the table.

I hope this session is clear and i hope this knowldge should help you in interviews.

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