SQL-lösning?

-löste sig-

Du skriver ej vilket DBMS, men om subqueries stödjs:

...
where local
   in ( select local
          from forum_headkat h
        inner
          join forum_underkat u
            on h.id = u.big_cat
        inner
          join forum_inlagg i
            on u.id = i.cat
         where i.id = <thread id> )