Oracle NVL function

The NVL function is used to replace NULL values by another value.

The syntax for the NVL function is:
NVL( value_in, replace_with )

value_in if the function to test on null values. The value_in field can have a datatype char, varchar2, date or number datatype.
replace_with is the value that is returned if value_in has a null value. The NVL statement works like this pl/sql code:

if (value_in is NULL) then
  return replace_with;
else
  return value_in;
end if;

Sample code: 

select nvl(salary, 0)
from   employees;

select nvl(ref_code,'Unknown')
from   users;