NullValue is used together with NullAsValue. All null values in the fields listed after NullAsValue syntax will be replaced with the string set in the variable NullValue.

This is very useful when we want to make null values searchable. For example, if we want to easily spot how many customers have not entered a phone number, we can create something like the following example:

NullAsValue Telephone;

SET NullValue='%NULL%';

Customers:
LOAD 
    Customer
    ,Telephone
 FROM [LIB://MyFilesCustomers.csv];