You can also use DECODE to create columns of summarized data in a single query.
NVL is a simple function that replaces a null value with the specified value. It is useful when you want to perform arithmetic on columns that might contain null values.
In these cases, a null value will cause the arithmetic to result in a null value.
To avoid this, you can substitute zero for null value using the NVL function. Values that are anything other than null remain unchanged. The syntax is shown below.