相邻列表和递归查询使用CTE，如何补回?

I have a query which produces flattened hierarchy from a parent-child table (self join, adjacent list). The problem is that this query produces NULLs for levels which does not have any child. Now my intention is to 'backfill' these levels to produce a table which does not contain any NULL values in level columns. How should I modify this query? I have a query which produces flattened hierarc