Ladislav Lenart wrote
> Hello all.
>
> I am curious about the following usage of CTEs:
>
> Imagine three tables:
> * item (id, item_type1_id, item_type2_id, ...)
> * item_type1 (id, ...)
> * item_type2 (id, ...)
> where
> * item_type1_id is FK to item_type1 (id)
> * item_type2_id is FK to item_type2 (id)
The mental model that comes to mind is:
item (item_id)
item_type1 (type1_id, item_id <FK>)
item_type2 (type2_id, item_id <FK>)
Or even better:
item (item_id <PK>)
item_type1 (item_id <PK; FK>)
item_type2 (item_id <PK; FK>)
You'd need a trigger on these tables if you want to enforce the "only a
single type allowed" restriction but otherwise this model is much more
usual.
The other mental hang-up with your model is that the "item_type1_id" has a
one-to-one relationship with the item_type1 table so that deleting the item
means it is OK now to delete the associated type. This is unusual.
David J.
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