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The User (Paul) has defined his Requirements in the following terms ...
A wiring harness database is required.
At the top are the cable(s).
Each cable has two or more plugs.
Each cable has multiple wires.
Each wire can be a different color and different size (wire gauge).
And then some fool splices in a wire that goes to three or more plugs.
Each of the plugs could be a different type !
Additional details were subsequently provided :-
Plugs have "Pins" that the wires go into.
These pins are usually numbers or could be letters and not all pins have wires.
Optionally Plug_types table could also have a field that lists the number of
pins for that plug type.
This could be used to validate wiring later in the process.
Therefore all plugs have pins and some pins have wires.