sql - 如何从类别表中以分层形式输出所有类别和子类别？

Question

我有一个包含列 id、name、level、parent_id 的类别表。我有一个产品类别，例如 .. Jewellery and watches > Jewellery > Ear-rings。

我的分类看起来像

id, name, level, parent_id
1, Jewellery and watches, L1, null
2, Jewellery, L2, 1,
3, Ear-rings, L3, 2

我如何编写一个会给我以下输出的查询

id, name, displayName
1, Jewellery and watches, Jewellery and watches
2, Jewellery, Jewellery and watches > Jewellery
3, Ear-rings, Jewellery and watches > Jewellery > Ear-rings

Answer 1

使用递归查询。

with recursive top_down as (
    select id, parent_id, name, name as display_name
    from categories
    where parent_id is null
union all
    select t.id, t.parent_id, t.name, concat(display_name, ' > ', t.name)
    from categories t
    join top_down r on t.parent_id = r.id
)
select id, name, display_name
from top_down;

 id |         name          |                 display_name                  
----+-----------------------+-----------------------------------------------
  1 | Jewellery and watches | Jewellery and watches
  2 | Jewellery             | Jewellery and watches > Jewellery
  3 | Ear-rings             | Jewellery and watches > Jewellery > Ear-rings
(3 rows)

请注意，level 列基本上是多余的。