-- correct
with recursive
    tree (NAME) as (
      select r.rdb$relation_name from rdb$relations r
      union all
      select r2.rdb$relation_name || tree.NAME from rdb$relations r2, tree
      where 1 = 0
    )
select * from tree

-- incrorrect
with recursive
    tree (NAME) as (
      select r.rdb$relation_name from rdb$relations r
      union all
      select r2.rdb$relation_name || tree.NAME from rdb$relations r2, tree
      where 1 = 0
    ),
    tree_2 as (
        select c.rdb$character_set_name from rdb$character_sets c
        union all
        select c2.rdb$character_set_name from rdb$character_sets c2, tree_2
        where 1 = 0
    )
select * from tree, tree_2

-------------
Dynamic SQL Error.
SQL error code = -206.
Column unknown.
TREE.NAME.
At line 5, column 43.