Sams Teach Yourself SQL in 21 Days, Fourth Edition

Using the DUAL Table

The DUAL table is a dummy table that exists in every Oracle database. This table is composed of one column called DUMMY whose only row of data is the value X. The DUAL table is available to all database users and can be used for general purposes, such as performing arithmetic (where it can serve as a calculator) or manipulating the format of the SYSDATE.

SQL> desc dual;

 Name                            Null?    Type
 ------------------------------- -------- ----
 DUMMY                                    VARCHAR2(1)

SQL> select *
  2  from dual;

D
-
X

Take a look at a couple of examples using the DUAL table:

SQL> select sysdate
  2  from dual;

SYSDATE
---------
08-MAY-97

SQL> select 2 * 2
  2  from dual;

      2*2
---------
        4

Pretty simple. The first statement selects SYSDATE from the DUAL table and ...