Difference between Primary Key and Unique Key

Hi Friends,

Difference between Primary Key and Unique Key is a very key question and it is generally asked in interview and in exams as well. I have listed the difference between Primary Key and Unique key below:-

Primary Key

1) Can not contain NULL values, this is because when we create a primary key, the NOT NULL constraint is automatically created for the column(s) the primary key is set. Note:- Unique key constraint is also created automatically therefore Primary Key is a combination of Unique Key constraint and NOT NULL constraint.
2) We can have only one Primary Key per table.
3) Creates clustered index by Default. We can have only one clustered index.
4) Primary key is used to uniquely identify a row or record in a table. Primary key ensures that no duplicate rows exist in a table.

Unique Key

1) Can or can’t contain NULL values i.e. if we define unique key on a column which is not null , then this unique key can also be used as an alternate primary key functionality to identify unique rows in a table. Note: - The number of NULL values a column can contain depends on RDBMS we are using i.e. SQL Server and DB2 don’t allow more than one NULL values whereas in Oracle we can have more than one NULL value.
2) We can have more than one Unique Key per table
3) Creates non-clustered index by Default
4) Unique key constraint ensures that no two rows contain duplicate value in a column or field, but if it is used with NOT NULL column then this can also serve the purpose of Primary key or can be used as alternate primary key.

Hope this will help you in understanding the concept of Primary Key and Unique Key. If you still have some doubt or needs more information you are always welcome.

With Regards,
Arvind

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Difference between Primary Key and Unique Key

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