SQL NVL Function Explained: Replace NULL Values

In SQL, the NVL function is used to check if a field or expression is NULL. If it is NULL, it returns a specified default value; if it is not NULL, it returns the value of the field or expression itself. For example, the syntax of the NVL function is as follows:

NVL(expression, default_value)

The expression is the field or expression that needs to be checked for NULL, while the default_value is the value returned when the expression is NULL. This function is frequently used when dealing with query results to prevent errors and unnecessary logic processing caused by null values.