Com relação às tabelas cliente e visita, descritas anter

delete from cliente

where not exists

   (select * from visita v

      where v.codigo in cliente.codigo)

delete from cliente

where not exists

    (select * from visita v

       where v.codigo <> cliente.codigo)

delete from cliente

where codigo in (select codigo from visita)

 delete from cliente

where codigo <> ALL (select codigo from visita) 

delete from cliente

where (select count(*) from visita v

              where v.codigo <> cliente.codigo) > 0