What is the difference between primary, unique and foreign key constraints, and indexes?

Question

What is the difference between primary, unique and foreign key constraints, and indexes?

I work on Oracle 10g<

Answer 1

1)A primary key is a set of one or more attributes that uniquely identifies tuple within relation.

2)A foreign key is a set of attributes from a relation scheme which can be uniquely identify tuples fron another relation scheme.