if its behind a form, use an IF statement. if its in a control source of a control, use the IIF() statement. if you don't know the syntax, look it up. its in the help menu.
It really depends where you are performing the test. You have two basic choices (VBA and SQL). Since you are using IIf() in your example, I will deal with the SQL answer. If you want/need the VBA one, that can be address in a later post.
Looking up the DEFINITION of an IIf() statement, you find the following:
IIf( { Condition to test }, { Value or Expression if Condition=True }, { Value or Expression if Condition=False } )
The "Value or Expression if Condition=True" can be another IIf() statement, and there are multiple levels allowed. Since this has been discussed in these Forums many times, I invite you to search these Forums for additional details.
Note: As usual, Work interrupted me in my attempt to respond, and others replied with similar responses before I did. All three of us agree as to the resolution to your issue, so I leave it to you to find the best way to make it work for your project.