Request QUERY for non-assigned values

Dear all:
I have a field named "SOURCE" which is represented with a scroll bar and options "End User", "Reseller", "Distributor", and "Tier One Customer". However, sometimes I do not pick any of them into SOURCE.

It occurs to me that when no values are assigned to SOURCE, I will not be able to QUERY those data IF my condition is more than one.

For instance:

WHERE ((([Quote Details].QuoteNumber) Like "*" & [Enter a Quote Number: ]) AND (([Quote Master List].Customer) Like "*" & [Enter a Customer: ] & "*") AND (([Quote Master List].source) Like "*" & [Enter a SOURCE: ] & "*") AND (([Quote Details].PartNumber) Like "*" & [Enter Part Number: ] & "*"))

Three windows will be prompted to ask me to "Enter a Quote Number", "Enter a Customer", "Enter a source", and "Enter a part number".

If I just enter 12005 for "quote number" and nothing for the rest three, it will list out all information for quote 12005. However, it DOES NOT SHOW those data where SOURCE is not assigned with a value.

On the other hand, if I just have a QUERY for QuoteNumber ONLY, and I enter 12005 at the prompt, then I will be given all information that links to 12005, including the ones that has SOURCE with empty blank (no assigned values).

In this case, my purpose of doing a multi-field matching query is incomplete. I am not sure in MS Access, when you have a scroll bar of values but you do not assign anything to it, is it considered as an empty string or as a NULL character or?



Any of you have some idea of how to fix this situation? I tried Enter a SOURCE: ] & "*" & (IsNULL) but it does not seem to work as well....

anyone...? =__=

You can see in this link :h77p://www.microsoft-accesssolutions.co.uk/blank_query_parameter.htm