Selecting from the DUAL Table
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" for many examples of selecting a constant value from DUAL.
Note:
Beginning with Oracle Database 10
g Release 1, logical I/O is not performed on the
DUAL table when computing an expression that does not include the
DUMMY column. This optimization is listed as
FAST DUAL in the execution plan. If you
SELECT the
DUMMY column from
DUAL, then this optimization does not take place and logical I/O occurs.
