select t.kert,
       t.x3,
       t.e1,
       t.e2
  from (
select (k.x1 + k.x2) as kert, k.x3, z.e1, z.e2
  from test1 k
  join test2 z on (z.e1 = k.x1)
  )
  where t.kert > 0 

select k.x1 + k.x2 as kert, k.x3, z.e1, z.e2
  from test1 k
  join test2 z on z.e1 = k.x1
  where k.x1 + k.x2 > 0