分类表：
=# \d
List of relations
Schema | Name | Type | Owner
--------+-------------+-------+-------
public | categories | table | pgsql
public | products | table | pgsql
public | ticketlines | table | pgsql
(3 rows)
分类内容：
=# select * from categories;
id | name | parentid
----+--------+----------
1 | Rack |
2 | Women | 1
3 | Shorts | 2
4 | Wares |
5 | Toys | 4
6 | Trucks | 5
(6 rows)
运行以下查询：
WITH RECURSIVE nodes_cte(name, id, parentid, depth, path) AS (
-- Base case?
SELECT c.name,
c.id,
c.parentid,
1::INT AS depth,
c.id::TEXT AS path
FROM categories c
WHERE c.parentid = ''
UNION ALL
-- nth case
SELECT c.name,
c.id,
c.parentid,
n.depth + 1 AS depth,
(n.path || '->' || c.id::TEXT)
FROM nodes_cte n
JOIN categories c on n.id = c.parentid
)
SELECT * FROM nodes_cte AS n GROUP BY n.name, n.id, n.parentid, n.depth, n.path ORDER BY n.id ASC
;
产生这些结果：
name | id | parentid | depth | path
--------+----+----------+-------+---------
Rack | 1 | | 1 | 1
Women | 2 | 1 | 2 | 1->2
Shorts | 3 | 2 | 3 | 1->2->3
Wares | 4 | | 1 | 4
Toys | 5 | 4 | 2 | 4->5
Trucks | 6 | 5 | 3 | 4->5->6
(6 rows)
伟大的！
但给定一个类似的表（类别）：
=# \d categories
Table "public.categories"
Column | Type | Modifiers
----------+-------------------+-----------
id | character varying | not null
name | character varying | not null
parentid | character varying |
image | bytea |
Indexes:
"categories_pkey" PRIMARY KEY, btree (id)
"categories_name_inx" UNIQUE, btree (name)
Referenced by:
TABLE "products" CONSTRAINT "products_fk_1" FOREIGN KEY (category) REFERENCES categories(id)
=# select * from categories;
id | name | parentid | image
--------------------------------------+-------+--------------------------------------+-------
611572c9-326d-4cf9-ae4a-af5269fc788e | Rack | |
22d15300-40b5-4f43-a8d1-902b8d4c5409 | Women | 611572c9-326d-4cf9-ae4a-af5269fc788e |
6b061073-96f4-49a1-9205-bab7c878f0cf | Wares | |
3f018dfb-e6ee-40d1-9dbc-31e6201e7625 | Toys | 6b061073-96f4-49a1-9205-bab7c878f0cf |
(4 rows)
相同的查询产生零行。
为什么？
它与主键/外键有关吗？