DB+-+Keys

=Foreign Keys and Primary Keys=

A key in which it is not a primary key in its own table is called a foreign key. Although the key may be a primary key in another table. A **foreign key** is a relationship or link between two tables which ensures that the data stored in a database is consistent. The foreign key link is set up by matching a column in one table to the primary key column in another table. The primary key is one of the unique keys defined on a table, but is selected to be the key of first importance. There can be only one primary key on a table.
 * DVD Code || Title || Cost || Date Out || Date Due ||
 * 002 || Fargo || £2.50 || 03/09/04 || 04/09/04 ||
 * 003 || American Pie || £2.50 || 01/09/04 || 02/09/04 ||
 * 008 || Reservoir Dogs || £2.50 || 04/09/04 || 06/09/04 ||
 * 011 || Notting Hill || £2.50 || 04/09/04 || 06/09/04 ||
 * 014 || Prime Suspect || £2.00 || 27/08/04 || 28/08/04 ||

The Primary Key is DVD Code.


 * Member Number || Member Name || Telephone Number || DVD Code ||
 * 1012 || Isobel Ringer || 293847 || 003 ||
 * 1034 || Jonh Silver || 142536 || 008 ||
 * 1056 || Fred Flintstone || 817263 || 011 ||

Whereas the Primary Key here is Member Number, and the Foreign Key is the DVD Code. These two tables are linked by the DVD Code.