Hi fabio,


>------------
>

>SELECT genus, species, continent ;
>    FROM Species P INNER JOIN Site S ON P.Pkey = S.fkey AND S.continent IN ('Africa','Asia') ;
>    WHERE EXISTS (SELECT * FROM Site WHERE fKey = P.Key AND Continent = 'Asia') AND ;
>          EXISTS (SELECT * FROM Site WHERE fKey = P.Key AND Continent = 'Africa') ;
>    ORDER BY 1,2
>

>------------
>
>If i want a general solution, then:
>

>SELECT genus, species, continent ;
>	FROM Species P JOIN Site S ON P.Pkey = S.fkey AND S.continent IN (ContinentList) ;
>	WHERE P.Pkey IN (SELECT fkey FROM Site WHERE Continent IN (ContinentList) GROUP BY 1 HAVING COUNT(*)=ContinentListCount) ;
>	ORDER BY 1,2
>

Not bad... However, we only have 7 continents ;-) Seriously, though this indeed is a good example there are some difference to note:

1. You're example executes two queries internally where the subquery is not optimized, which can have a performance disadvantage when there are a lot of records in the Site table.

2. You're not able to do a query saying: Give me the species from 'Asia' and 'Afrika' that do not occur in 'Europe'

3. OTOH, you can answer queries like, give me the species that occur in four of five given continents.

Walter,