with t (x, a, b) as
  (
    select 0, 10, 20 from rdb$database union all
    select 0, 20, 10 from rdb$database union all
    select 10, 0, 20 from rdb$database union all
    select 10, 20, 0 from rdb$database union all
    select 20, 0, 10 from rdb$database union all
    select 20, 10, 0 from rdb$database union all

    select null, 10, 20 from rdb$database union all
    select null, 20, 10 from rdb$database union all
    select 10, null, 20 from rdb$database union all
    select 20, null, 10 from rdb$database union all    -- wrong 
    select 10, 20, null from rdb$database union all    -- wrong 
    select 20, 10, null from rdb$database union all

    select null, null, 20 from rdb$database union all
    select null, 20, null from rdb$database union all
    select 20, null, null from rdb$database union all

    select null, null, null from rdb$database
  ),

  cmp as
  (
    select t.*, x between a and b as cmp1, a <= x and x <= b as cmp2
      from t
  )

select * from cmp
 where cmp1 is distinct from cmp2