Answer Posted / suresh kumar

Hi, i give the definitions with easy understand exapmles

A primary key is used to uniquely identify each row in a
table. It can either be part of the actual record itself,
or it can be an artificial field (one that has nothing to
do with the actual record).
A primary key can consist of one or more fields on a table.
When multiple fields are used as a primary key, they are
called a composite key.

EX:If we take EMP table, Empno is the primary key and in
DEPT table, Deptno is the primary keys

A foreign key is a field (or fields) that points to the
primary key of another table.
The purpose of the foreign key is to ensure referential
integrity of the data.
In other words, only values that are supposed to appear in
the database are permitted

EX:In EMP table Deptno field is the foreign key because it
pointing to the Deptno in DEPT table

A Reference key is a field (or fields) that points to the
primary key of the same table

EX:In EMP table Mgr field is the Reference key because it
pointing to the Empno field in the same table

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