IsNull

Reports whether the value of a column or expression is null.

IsNull ( any )

Argument	Description
any	A column or expression that you want to test to determine whether its value is null

Boolean. Returns true if any is null and false if it is not.

Use IsNull to test whether a user-entered value or a value retrieved from the database is null.

This expression returns true if either a or b is null:

IsNull(a + b)

This expression returns true if the value in the salary column is null:

IsNull(salary)

This expression returns true if the value the user has entered is null:

IsNull(GetText())