Hello everyone
I'm having a problem, and I do not understand why.
I have a structure similar to:

with recursive
 aux as (
  select 0 as n
    from rdb$database
  union all
  select aux.n + 1
    from aux
    where aux.n <= 20
 )

 select aux.n, mytable.id
 from aux
   LEFT JOIN mytable on aux.n between mytable.id and mytable.id +1 
 where mytable.id is not null

Where the left join behaves as join.

Any condition that I put in the clause WHERE referred to mytable, converts the LEFT JOIN into JOIN