I have three tables: EARTAG table, LIVESTOCK table, and a joint table called EARTAGGING_EVENT. All available ear tags will be entered into the EARTAG table whenever they are purchased. During the ear tagging event the available ear tags will be mapped with livestock. I want to be able to assign ear tags to livestock in the EARTAGGING table from the available ear tags through with a drop down list from a dwindling value list so that I don't assign used ear tags. Once an ear tag has been assigned it should not be available in the dropdown list. This will prevent me from assigning one ear tag to more than one livestock. When I am in the Eartagging layout, I want the ear tag dropdown list to dwindle as it is being assigned.