The DUAL table is a special one-row, one-column table present by default in all Oracle RDBMS installations.
The table has a single VARCHAR2(1) column called DUMMY that has a value of ‘X’.
Example 1:
SELECT * FROM DUAL;
returns:
DUMMY
--------
X
Example 2:
SELECT SYSDATE FROM dual;
returns:
15-JUN-2015 17:16:00