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

Key/index : A key is an aspect of a LOGICAL database design, an index is an aspect of a PHYSICAL database design. A key corresponds to an integrity constraint, an index is a technique of physically recording values that can be usefully applied when enforcing those constraints.

Primary/foreign : A "primary" key is a set of attributes whose values must form a combination that is unique in the entire table. There can be more than one such set (> 1 key), and the word "primary" is a remnant from the earlier days when the designer was then forced to choose one of those multiple keys as being "the most important/relevant one". The reason for this was primarily in combination with foreign keys :

Like a "primary" key, a "foreign" key is also a set of attributes. The values of these attributes must form a combination that is an existing primary key value in the referenced table. I don't know exactly how strict this rule still applies in SQL today. The terminology has remained anyway.

Unique : keyword used to indicate that an index cannot accept duplicate entries. Unique indexes are obviously an excellent means to enforce primary keys. To the extent that the word 'unique' is used in contexts of LOGICAL design, it is superfluous, sloppy, unnecessary and confusing. Keys (primary keys, that is) are unique by definition.