DUAL table

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'.

Example use

Oracle's SQL syntax requires the FROM clause but some queries don't require any tables - DUAL can be readily used in these cases.

SELECT 1+1
FROM DUAL;
 
SELECT SYSDATE 
FROM   DUAL;
 
SELECT USER 
FROM   DUAL;

History

The DUAL table was created by Charles Weiss of Oracle corporation to provide a table for joining in internal views:

I created the DUAL table as an underlying object in the Oracle Data Dictionary. It was never meant to be seen itself, but instead used inside a view that was expected to be queried. The idea was that you could do a JOIN to the DUAL table and create two rows in the result for every one row in your table. Then, by using GROUP BY, the resulting join could be summarized to show the amount of storage for the DATA extent and for the INDEX extent(s). The name, DUAL, seemed apt for the process of creating a pair of rows from just one.^[1]

The original DUAL table had two rows in it (hence its name), but subsequently it only had one row.

Optimization

DUAL was originally a table and the database engine would perform disk IO on the table when selecting from DUAL. This disk IO was usually logical IO (not involving physical disk access) as the disk blocks were usually already cached in memory. This resulted in a large amount of logical IO against the DUAL table.

Later versions of the Oracle database have been optimized and the database no longer performs physical or logical IO on the DUAL table even though the DUAL table still actually exists.

In other database systems

MySQL allows DUAL to be specified as a table in queries that do not need data from any tables.^[2]

Notes

1. ^ Oracle Magazine article describing the history of DUAL
2. ^ http://dev.mysql.com/doc/refman/5.0/en/select.html

External links

• Oracle Database 10g: The Top 20 Features for DBAs