Dual table:

• DUAL table is a dummy table which contains one row and one column and by default it is present in database.It contains a single VARCHAR2(1) column known as DUMMY with a value "X".
• It can be accessed by all users but the owner of DUAL table is SYS. We can use this table to perform some calculation and we can also display system date.
• It is used when we are interested in result only.

We can see the structure of the DUAL table using the following query:

DESC DUAL; 

Output will be like this:

Name                 Null?   Type
--------------------------- ------
DUMMY                        VARCHAR2(1)

We can see the data of dual table using the following query:

SELECT * FROM DUAL;  

Output will be like this:

DUMMY
----------
X

Query to check the system date using dual table:

SELECT sysdate FROM DUAL ; 

Output will be like this:

SYSDATE
---------
25-JAN-16

Query to perform some calculation using dual table:

SELECT 25+10-25*25/25 FROM DUAL;  

Output will be like this:

25+10-25*25/25
---------------------
10.0000