How to exclude a @variable in SQL query if a control does not contain a value

I have a search page with three dropdownlist controls and a search button. A simple select query to retrieve data in a datalist control is on the same web page. I like to filter data based on the three dropdownlist controls. In the filter column of my query I put a variable like this @controlname. One of the dropdownlist controls does not require input. If I put a variable for the field in the query I get an error message "...expects the parameter @controlname, which was not supplied". I have tried isnull(@controlname) did not work. How can I execute the query if no input in this particular control then show alle records based on the other control variables?
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Change the select criteria to
LIKE isnull(@controlname,'%')

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Guy Hengel [angelIII / a3]Billing EngineerCommented:
please show the relevant code.
PdeLormeAuthor Commented:
This is the part of the where clause in my SQL statement. The SQL statement will be used as objectdatasource in my datalist control.

WHERE     (GuestID = @AantalGuests) AND (RoomID = @AantalRooms) AND (City = @CityAirport)

The variable @AantalGuests must be optional. If no value is selected in the dropdownlist, it should not be part of the filter on the table content. Btw, LIKE isnull(@controlname,'%') gives an error "data type error in expression". Hope you find a solution!
Guy Hengel [angelIII / a3]Billing EngineerCommented:

WHERE     (GuestID = @AantalGuests OR  @AantalGuests  IS NULL) AND (RoomID = @AantalRooms) AND (City = @CityAirport)

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PdeLormeAuthor Commented:
Works like a charm. Thank you!
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Microsoft SQL Server

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