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.
