[GRASS5] Layers Clarification

[Radim Blazek]

On 3/7/06, twiens <twiens at interbaun.com> wrote:
> > >> I want to embellish on Radim's response a bit.
> > >>
> > >> ONE vector object can join with can be joined with ONE
> > attribute table for >> EACH "layer" it has. If it has ONE
> > layer, it can join with ONE table, if it >> has TWO layers
> > , it can join with TWO tables. >>
>
> Michael,
>
> In Radim's example of v.clean
>
>     1
> +-----------+
>               2
>         +----------------+
>
> v.clean yielding
>
>   1      1,2   2
> +------+----+------------+
>
> According to Radim's text the result is still in one layer.
>
> Thus I must conclude that although GIS objects can only
> connect to one table per layer, GIS objects can connect to
> more than one row in the linking table thus supporting many
> to one joins as well as one to one or one to many.

Yes. One geometry object can be linked to more rows in attribute table.
In that case the geometry object has more categories (the same layer)
each pointing to one row in the attribute table. One category can only
link to one row, no more.  The relation geometry-table can be
one-to-many. The relation category-table can only be
on-to-one or many-to-one.

It is probably much easier  get idea from a picture:
   http://mpa.itc.it/radim/g51/bridge.png
It covers all possible cases I think.

Radim

> This seems odd. What am I missing here.
>
> T
>