NVL evaluates expression1. If expression1 is
not NULL, then NVL returns the value of expression1.
If expression1 is NULL, NVL returns the value of expression2.
The expressions expression1 and expression2 can be of
any data type, as long as they can be cast to a common compatible
data type.
Suppose that the addr column of the employees table
has NULL values in some rows, and the user wants to be able to print
the label Address unknown for these rows. The user
enters the following SELECT statement to display the label Address
unknown when the addr column has a NULL value:
SELECT fname, NVL (addr, 'Address unknown') AS address
FROM employees;