Difference between primary key and unique key

Please log in or register to like posts.

Unique Key: It’s a column or a group of columns that can identify uniqueness in a row.
Primary Key: It’s also a column or group of columns that can identify uniqueness in a row.
The Primary key is just another name for the unique key, but the default implementation in SQL 

The server is different for Primary and Unique Key.

Primary Key:

  • There can only be one primary key in a table.
  • In some DBMS it cannot be NULL.
  • Primary Key is a unique key identifier of the record.
Unique Key:

  • It can be more than one unique key in one table.
  • The unique key can have valuesNULL.
  • It can be a candidate key.
  • The unique key can be andNULL may not be unique.

By Default:
  1. Primary Key creates a Clustered index and the Unique key creates a Non-Clustered Index.
  2. PK is not null, but Unique Key allows nulls (Note: By Default)
  3. There can only be one and only one PK on a table, but there can be multiple UK’s
  4. You can override the default implementation depending upon your needs.
It really depends on what is your aim when deciding whether to create a UK or PK. It follows an analogy like “If there is a team of three people, so all of them are peers, but there will be one of them

who will be a pair of peers: Primary Key and Unique Key have a similar relation.”. I would suggest reading this article

: The example given by the author may not seem suitable, but try to get an overall idea.


Already reacted for this post.


One comment on “Difference between primary key and unique key

Leave a Reply

Your email address will not be published. Required fields are marked *