RELATIONAL DATABASES:

Relational databases are widely used in industry. They presuppose that all information can be represented in named tables each having a fixed number of named columns and a varying number of rows. Each row contains information about a separate entity, having the properties described in the columns (fields) of that row.

For example a computerized library system might include the rows shown below in a relational table named library:

Table: Library

Author	Title	Date	Type
Shakespeare	The Tragedy of Hamlet Prince of Denmark	1600	play
Shakespeare	The Tragedy of King Lear	1610	play
Doyle	The Hound of the Baskervilles	1890	story
Conrad	Lord Jim	1900	story

Rows may be related to other rows (or excluded from such relationships) using the information contained in these rows.

Table: Events

Date	Event
1598	End of French Wars of Religion
1600	English East India Company Founded
1600	Tokugawa Period begins in Japan
1602	Dutch East India Company Founded
...	...

For example, if the table of events, shown above, existed: then it would be possible to construct a new table Coincidences by relating library rows to event rows which had common dates as shown below:

Table: Coincidences

Title	Date	Event
The Tragedy of Hamlet Prince of Denmark	1600	English East India Company Founded
The Tragedy of Hamlet Prince of Denmark	1600	Tokugawa Period begins in Japan
...	...	...

Note that some columns from the original tables (for example AUTHOR) have been deliberately discarded in the result above.

In addition to being able to select rows and columns from a relational database, rows can be added, updated and deleted, and table may be created, altered, or deleted. Database systems provide many other features such as security, data integrity, concurrency, and data recovery in the event of system failures, etc.