Difference between primary key and unique key

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.

http://tsqltips.blogspot.com/2012/06/difference-between-unique-key-and.html