Difference Between Primary Key and Unique Key

By Mohit Uniyal|Updated : June 9th, 2022

Difference Between Primary Key and Unique Key: Keys are widely used to construct the relationship and access information between two or more tables. The keys are used to provide uniqueness to the columns in tables. The major difference between primary key and unique key is that the unique key is used to prohibit the duplication of the entries in the columns whereas the primary key is used to identify. 

Before learning about the difference between primary key and unique key, we will elaborate on what is the primary key and unique key. In the sections below, various parameters are explained regarding the differences between the two.

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Difference Between Primary Key and Unique Key

Although there are various similarities between the primary key and the unique key such as used to create the uniqueness and integrity of the database stored in the column. There are a few differences between the two which we will discuss here. Check out the table provided below to know the difference between primary key and unique key in various aspects:

Key Differences Between Primary Key and Unique Key

Primary KeyUnique Key
This is used to prohibit null values.Do not prohibit null values.
It is known as the unique identifier.Used for uniques identification in place of the primary key. 

Only once can be used in the table.

Can be used more than once on a table.
A clustered index is created. No clustering of the index is created using this key.
Enforces the integrity of the entity.It does not enforce the integrity of the entity.
Values can not be deleted.Values can be modified.

What is a Primary Key?

In a table, to identify every row a Primary key is used. It is defined at a column level. The primary key is used when null values are to be restricted to be entered in a column. They can also be used to maintain the integrity of the database in the column.

The key feature of the primary key is to implement the integrity of the elements in the table. The columns where the primary key is used, are defined as not null. More than one primary key can not be used in a table. The primary key can contain more than one column per table. 

What is a Unique Key?

A unique key is used to uniquely identify the database in one or more columns in a table. It is similar to the primary key but the null value is not restricted in this. Null values can enter the column of the table. Identical values are not stored in this. 

A unique key is used to examine whether the entries in the columns are unique or not. It will be used instead of primary when a null value is allowed to be inserted. The key feature of a unique key is that more than one of it can be defined for the table.

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FAQs on Difference Between Primary Key and Unique Key

  • The major difference Between Primary Key and Unique Key is based on the allowance of the entry of null entry. The primary key does not allow null entries whereas the unique key allows null entries.

  • A primary key is used to identify each and every element of the rows in a table. Only allows unique elements or entries. It does not allow null entries to be filled.

  • Unique key is another method of identifying database in the table. They are used when primary key is not availaible. They allows the entries of the null values. The unique key can be used more than once.

  • As per the number of usage in the table, the difference between primary key and unique key is that primary key is used only once whereas unique key can be used more than once in a table.

  • As per the creation of index, the main difference between a primary key and unique key is that primary key creates cluster whereas unique key does not create any cluster of index.

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