What is the difference between Primary Key and Unique key?

[chinmay]

Primary Key: A column in a table whose values uniquely identify the rows in the table. A primary key value cannot be NULL. Unique Key: Unique Keys are used to uniquely identify each row in the table. There can be one and only one row for each unique key value. So NULL can be a unique key.There can be only one primary key for a table but there can be more than one unique for a table.

[Shivangi Panwar]

In Primary Keys :- We can have only one Primary key in a table.
In Unique kays :- We can have more than one unique key in a table.

[karthikaqpt]

Primary Key:

• Can be only one in a table
• It's also a column or group of columns that can identify a uniqueness in a row.

Unique Key:

• Can be more than one unique key in one table
• It's a column or group of columns that can identify a uniqueness in a row.

[mannai]

According to my knowledge both primary key and Unique key is almost the same. but the only difference is that in a database we can only have one Primary key but we can have n number of unique keys according to the need.

[Digitalpix]

primary key is a unique identifier in a relational Database..
unique key is a super key & Uniquely identifies each record in a database ....

[ziya.pathan]

We can use only one Primary key in a table.
We can use more than one unique key in a table.

[luffy268]

Both Primary Key & Unique Key should have unique values.


1. The only difference is Unique Key column can have one row with NULL value.


2. And you can create only 1 PK, and multiple UK.


It also depends upon Database product. Like in SQL Server, upon creating a PK it created a Clustered Index by default on a table. But Unique Key cerates a Non-Clustered Index on the table column.