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Primary key and Unique key are two important concept in relational database, and used to uniquely identify a row in a table. Both primary key and unique key can identify a row uniquely but there are some subtle difference between them which we will see in this article. In fact primary key vs unique is a popular SQL interview questions along with classics like truncate vs delete and  How to manage transaction in database, mostly asked to fresher and 2 to 3 years experience guys in any programming language. SQL is not just limited to any DBA or PLSQL developer but its an important skill even for Java programmer and you can expect SQL interview question even in many Java interviews. Some time programmer also confuse between foreign key and unique key, which is primary key of other table in relation. 

Difference between primary key and unique key in SQL

As I said both primary and unique key uniquely identifies each row in table but there are some subtle difference between them. here are some of them :

1) Unique key in a table can be null, at-least one but primary key can not be null in any table in relation database like MySQL , Oracle etc.

2) Primary key can be combination of more than one unique keys in same table.

3) There can be only one primary key per table in relation database e.g. MySQL, Oracle or Sybase but there can be more than one unique key per table.

4) Unique key is represented using unique constraint while primary key is created using primary key constraint in any table and it’s automatically gets unique constraint.

5) Many database engine automatically puts clustered index on primary key and since you can only have one clustered index per table, its not available to any other unique key at same time.

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