mysql-forespørgsel join/inner join

Følgende skal vælge det, du beskriver:

select a.topic
from tableA a
join tableB b on b.id = a.userid
where b.location = 'Australia' -- or whichever location you filter on

hvilket svarer til:

select a.topic
from tableA a
join tableB b on b.id = a.userid and b.location = 'Australia'