Criteria field in query set to "no selection"

[4thangel]

I created in the criteria field of a query an conditional statement that should give as result "all entries allowed" when the form field (combobox c1) I refer to is isnull



SELECT Tbl_dara_test.product
FROM Tbl_dara_test
WHERE (((Tbl_dara_test.product)=IIf(IsNull([Forms]![f1]![c1]),(Tbl_dara_test.product) Like "*",[Forms]![f1]![c1])));

The - like "*" - is not working.
I tried also "*", "" and the same negative result

While if I create a text box (t1) in the form with a function =IIf(IsNull([c1]),"*",[c1]) and use in the query
SELECT Tbl_dara_test.product
FROM Tbl_dara_test
WHERE (((Tbl_dara_test.product) Like ([Forms]![f1]![t1])));

It works!!

Creating in the from an extra text field seems to me a dirty solution.
Is there a possibility to create a funtion in the query only?

[ssanfu]

"The Access Web" site has an example of using a parameter to return all records if Null

http://access.mvps.org/access/queries/qry0001.htm

[4thangel]

Veni, Vidi, Velcro a bit longer please.
Excellent answer. It works perfectly.

Thanks