The NVL() function is available in Oracle, and not in MySQL or SQL Server. This function is used to replace NULL value with another value. It is similar to the IFNULL Function in MySQL and the ISNULL Function in SQL Server.
For example, if we have the following table,
Store A 300
Store B NULL
Store C 150
The following SQL,
SELECT SUM(NVL(Sales,100)) FROM Sales_Data;
returns 550. This is because NULL has been replaced by 100 via the ISNULL function, hence the sum of the 3 rows is 300 + 100 + 150 = 550.