Oracle FAQ

What is Dual?

Dual is a constant table which always has one tuple and one column. It is frequently used as a testing table, to test expressions.

What is Oracle System Catalog?

The Oracle system catalog is comprised of tables and views which allow the user to obtain information about the database, such as what other tables and views are available, their attributes, constraints, etc. Oracle has hundreds of available system catalog relations, only some of which are available to the average user (as many are intended for use only by qualified database administrators).