oracle dual表

     DUAL is a table automatically created by Oracle Database along with the data
dictionary. DUAL is in the schema of the user SYS but is accessible by the name DUAL
to all users. It has one column, DUMMY, defined to be VARCHAR2(1), and contains one
row with a value X. Selecting from the DUAL table is useful for computing a constant
expression with the SELECT statement. Because DUAL has only one row, the constant
is returned only once. Alternatively, you can select a constant, pseudocolumn, or
expression from any table, but the value will be returned as many times as there are
rows in the table. Refer to "About SQL Functions" on page 5-2 for many examples of
selecting a constant value from DUAL.

ps:refer from book<<Oracle Database SQL Language Reference>>