Re: problem with table structure

Miguel Vaz  wrote:

> [...]
> * sites (generic):

> id_site
> name
> description
> x
> y

> * site_natural
> id
> id_site
> altitude

> * site_arqueology
> id
> id_site
> id_category
> id_period

> But i seem to be missing something. How can i have this in a way that its
> easy to list only "arqueology sites" for example. I feel the solution is
> simple enough, even for me, but its eluding me. Any help in the right
> direction would be very appreciated.

You mean archaeological sites that are not also natural
sites?

| SELECT * FROM site_arqeuology
|   WHERE id_site NOT IN
|     (SELECT id_site FROM site_natural);

There are numerous other ways to do this, i. e., with "LEFT
JOIN", "EXCEPT", etc.

Tim