Posted by othellomy on 03/06/07 08:58

On Mar 4, 11:00 am, bbcr...@gmail.com wrote:
> I have three tables with a relationship I've never worked with
> before. Can anyone suggest/comment on the best way to create a third
> normal form relationship between these tables?
>
> The tables basically are:
>
> TRAIN (TRAIN_ID and 15 columns about train specs, etc)
> TRUCK (TRUCK_ID and 12 columns about truck specs, etc)
> TRANSPORTATION_ITEM
> This table has, among others, two columns, TRUCK_ID and TRAIN_ID. If
> the truck column is used there can be no data in the train column and
> vice versa.
>
> This relationship seems denormalized to me, but I don't remember how
> to normalize it. Does anyone know the correct name for this kind of
> relationship?
>
> thanks!

You can have two extra columns. For transportation_item table one
column will be source with values 'truck' or 'train'. The other column
is id with values for truck_id or train_id.

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