NULL is a special value representing "no value" in all data types. NULL can be used on in operations like other values. But most operations has special rules when NULL is involved. The tutorial exercise below shows you some examples:

SET NULL 'NULL'; -- Make sure NULL is displayed

SELECT NULL FROM DUAL;

N

-

N

U

L

L

SELECT NULL + NULL FROM DUAL;

NULL+NULL

----------

NULL

SELECT NULL + 7 FROM DUAL;

NULL+7

----------

NULL

SELECT NULL * 7 FROM DUAL;

NULL*7

----------

NULL

SELECT NULL || 'A' FROM DUAL;

N

-

A

SELECT NULL + SYSDATE FROM DUAL;

NULL+SYSD

---------

NULL