ISNULL

Checks the value for a specified field and if the value is NULL it will replace the NULL value with the value supplied in the field name or constant value argument.

ISNULL ( <field_name>, <field_name or constant_value> )

field_name: The field to check the value of.
Field Name or Constant Value: The value to replace null with

Example: create a field at the CUSTOMER table level that combines the First Initial, Middle Initial and Surname. Some records will have null values for the Middle Initial. To prevent these records returning a NULL value the following syntax should be used.

Customer.FIRST_INITIAL + " " + isnull([Customer.SECOND_INITIAL], "") + " " + Customer.SURNAME