I needed the ability to set the visibility of a dropdown based on some other property of the user. Oh, and the dropdown needed to postback on the change of selection

Here is the solution I used:
[TotalPartnerships] is an integer returned in the main QUERY it is 0 or > 0.

As I am unaware of a way to 'disable' sub-queries to run or show I decided to use CSS to handle the visibility.

Notice this:
style="{IIF,[TotalPartnerships] > 1,,"display:none;"}"

Basically it says if the total partners is > 1 then show them and allow the user to select one. Otherwise don't show the selection.

Then notice this:
OnChange="form.submit()"

It handles the auto-postback


Here is the whole SubQuery Piece... it goes in the display.

<SELECT OnChange="form.submit()" name=cboPartnerID id=cboPartnerID style="{IIF,[TotalPartnerships] > 1,,"display:none;"}">
<OPTION Value="-1">ALL</OPTION>
{SUBQUERY, Name="", Query="SELECT PartnerID,PartnerName FROM Partners WHERE UserID = [*UserID]", Format="<option value= '[PartnerID]'>[PartnerName]</option>", SelectedFormat="<option value='[PartnerID]' SELECTED>[PartnerName]</option>", SelectedField="PartnerID",
SelectedItems="[cboPartnerID,Form]"}
</SELECT>

Of course you will need to have your own tables and select statements to make this work, however, the framework remains the same.