so this is working on the default recordset.
it sounds like you want to make the categories.CategoryLink part of the where clause:
WHERE categories.CategoryLink = WAQB_Param1 AND categories.CategoryLocation = WAQB_Param2
be optional
so change the where clause to:
WHERE (categories.CategoryLink = WAQB_Param1 OR -1 = WAQB_Param3) AND categories.CategoryLocation = WAQB_Param2
the create the variable as:
Name: WAQB_Param1
Type: Text
Default Value: -1
Runtime Value: $_GET['Category']
Name: WAQB_Param2
Type: Text
Default Value: -1
Runtime Value: $_GET['Department']
Name: WAQB_Param3
Type: Text
Default Value: -1
Runtime Value: $_GET['Category']