Difference between Primary Key and Unique Key

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Primary Key: There can only be one primary key in a tableIn some DBMS it cannot be NULLPrimary Key is a unique key identifier of the recordUnique Key:Can be more than one unique key in one tableThe unique key can have NULL valuesIt can be a candidate keyThe unique key can be NULL and may not be unique

Anonymous User · Answer

SQL Server, we have two keys which distinctively or uniquely identify a record in the database. Both the keys seems identical, but actually both are different in features and behaviours. In this article, I would like to share the key difference between primary key and unique key. For more help about keys in SQL Server refer the article Different Types of SQL Keys.

Difference between Primary Key & Unique Key

Primary Key

Unique Key

Primary Key can't accept null values.

Unique key can accept only one null value.

By default, Primary key is clustered index and data in the database table is physically organized in the sequence of clustered index.

By default, Unique key is a unique non-clustered index.

We can have only one Primary key in a table.

We can have more than one unique key in a table.

Primary key can be made foreign key into another table.

Unique Constraint can not be related with another table's as a Foreign Key.

CREATE TABLE Employee 
 (
 Emp_ID int PRIMARY KEY,
 Emp_Name varchar (50) NOT NULL,
 Emp_Sal int UNIQUE, 
 Location int NULL
 )

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Difference between Primary Key and Unique Key

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