sql - 确定分层路径的子计数

Question

我有一个包含以下列的表格:

child_count | path
------------+-----
            | /
            | /a
            | /a/a
            | /a/b
            | /a/b/c
            | /b

目前只有路径栏有数据。我想知道一个 SQL(最好是 PostgreSQL)查询能够确定每个路径的子计数，如下所示:

child_count | path
------------+-----
2           | /
2           | /a
0           | /a/a
1           | /a/b
0           | /a/b/c
0           | /b

Answer 1

Andomar 在我编辑时打败了我，但我还是会发布我的解决方案，因为它使用了一些 PostgreSQL 特定的东西。

with path_elements as (
  select path, array_length(string_to_array(path, '/'),1) as element_count
  from path_table
)
select parent_path, count(child_path) 
from (
  select p.path as parent_path,
         c.path as child_path
  from path_elements p
    left join path_elements c 
              on p.element_count + 1 = c.element_count 
             and substring(c.path, 1, length(p.path)) = p.path
) t
group by parent_path
order by parent_path;