Difference Between Primary Key and Unique Key

A series of tables having rows and columns that store information that is retrieved or used as and when required are termed as data.  The data stored in the rows and columns is determined by the keys used in the table.


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Primary Key vs Unique Key

The difference between the Primary Key and Unique Key is that a primary key is used to uniquely identifies a row of a table, the process of unique identification of rows is termed as entity integrity, a table only one unique key while unique key places additional conditions to make rows unique other than the primary key, a table can have more than one unique key.

Primary key vs Unique key

A primary key constraint in a table is used to identify the rows of the table uniquely. In any data table, there is only one primary key. It does not take null values, also it does not allow duplicate keys.

A unique key of a table confirms that the data in each row and column is unique and not repeated. In the table, there are more than one unique key.


Comparison Table

Parameter of ComparisonPrimary KeyUnique Key
Basic functionThe basic function of a primary key is to uniquely identify a row in a table.The basic function of a unique key is to apply additional conditions to a row so that it becomes unique.
Number of keys in a tableA table has only one primary key.A table can have more than one unique key.
Null value acceptanceA primary key does not accept null values.A unique key can accept null values.
IndexingA primary key has a clustered default index.The default index in a unique key is non clustered.
Duplicate keysDuplicate keys are not allowed in a primary key of a table.Duplicate keys are allowed in a unique key of a table if one or more key parts are null.


What is Primary Key?

One of the most important keys of a data table is the primary key. The primary key is a key that confirms the uniqueness of the data table.

A primary key does not accept null values. The primary key also does not accept duplicate values.  The default index of the primary key is clustered.

The syntax for creating the primary key in the table is-                                                                          

CREATE TABLE Persons (ID  int NOT NULL, Lastname varchar(255) NOT NULL, Firstname varchar(255), Age int, PRIMARY KEY(ID));

primary key

What is Unique Key?

Another key of the data table is the unique key. The unique key in the data table identifies rows uniquely other than the primary key. The function of the unique key is to apply conditions to the rows that make them unique.

A unique key can accept null values. It also takes duplicate values in the table if and only if one or more than one key parts are null. The default index of the unique key is encountered.

The syntax of creating the unique key in the table is-

CREATE TABLE Persons (ID int UNIQUE.  Name varchar(255) NOT NULL Surname varchar(255), Age int, UNIQUE KEY(ID));

Main Differences Between Primary Key and Unique Key

  1. A primary key can be associated with other tables as a foreign key while a unique key can’t be associated as a foreign key.
  2. A primary key is always unique while a unique key is not necessarily a primary key.


  1. https://link.springer.com/article/10.1023/A:1006720022161
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