Oracle: What is a dual table and why is it used?

Answer 1

Dual is a table that contains a dummy column with a row 'X'. it is used to satisfy the requirements of a SQL syntax containing a From clause.

Answer 2

It's a sort of dummy table with a single record used for selecting when you're not actually interested in the data, but instead want the results of some system function in a select statement:

e.g. select sysdate from dual;

The DUAL table is a special one-row table present by default in all Oracle database installations. It is suitable for use in selecting a pseudocolumn such as SYSDATE or USER

The table has a single VARCHAR2(1) column called DUMMY that has a value of "X"

You can read all about it in http://en.wikipedia.org/wiki/DUAL_table