The table named DUAL is a small table in the data dictionary that Oracle and
user-written programs can reference to guarantee a known result. This table has one
column called DUMMY and one row containing the value X.

DUAL表
1. Oracle 及用户程序可以使用此表来获得一个确定的值
2. 此表只有一列名为 DUMMY；只有一行数据，值为 X