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 advantage to dual is the optimizer understands dual is a special one row, one column table -- when you use it in queries, it uses this knowledge when developing the plan.
SQL> desc dual
Name Null? Type
----------------------------------------- -------- ----------------------------
DUMMY VARCHAR2(1)
DUAL is always there, every user can access it, and it always has the same contents. It is suitable for use in selecting a pseudo column such as SYSDATE or USER.
>select * from dual
DUMMY
-----
X
