Question : How do I give user a choice to run a query filtered by either one group or showing all groups?

I'm very familiar with using a form to pass the criteria to a query to filter results for one group of records.

What I  am confused about is whether it's possible to allow the user to select all records or a group of records using a form and a single query.

The goal:
Create a Form that opens a Report based on a Query.
Query is filtered by the criteria passed through the Form.
The Form has a combo box that lists each Group to be sorted.

That much is easy....

What I want is, in addition, to have the option to run the report with all groups without creating a separate query and report just for that option.

Here is what I've tried, and I may be way of base here, which is why I'm asking for your assistance:
In the criteria area of the filtered field I added:
=IIf([Forms]![Form1]![Product Number]=Null,""Like " & ""*""",[Forms]![Form1]![Product Number])

The SQL View looks like:
WHERE (((ProductList.ProductID)=IIf([Forms]![Form1]![Product Number]=Null,"" Like " & ""*""",[Forms]![Form1]![Product Number])))

So far, the expression will work only when there is a value. If no value is given the query returns no records. I assumed no value = Null,... maybe not.  However, for testing, if I just input Like "*" into the criteria I do get all records like I wanted.

So, for some reason, the expression is not working for me or I may be going about this completely the wrong way.

Thank you for your assistance.
Jeremy

Answer : How do I give user a choice to run a query filtered by either one group or showing all groups?

use this

=IIf([Forms]![Form1]![Product Number] Is Null,[ProductID],[Forms]![Form1]![Product Number])

or this

=IIf(isnull([Forms]![Form1]![Product Number]),[ProductID],[Forms]![Form1]![Product Number])