Primary key & foreign key

[vermar]

just to add here unique key can contain null value, while primary key can not contain null value.

[shasi1986]

primary key is combination of both unique and notnull keys.
unique key is a key which is not accept any duplication.
foriegn key is a key which is act as a primary key to some other table.

[sunshine60india]

Set of fields in database taken collectively to identify any row uniquely is called Candidate key...
So Database has more than 1 Candidate key...

Primary key is the Candidate key choosen by the Database Designer...

Unique Key is the constraint that is used to insert unique value in the field of the database...