Question

I am trying to do something that should be simple but I’m missing something. I am trying to return some hierarchical data in grouped and ordered. a Rather than trying to explain, I am attaching a picture… heard they are worth a thousand words:

Note that hard coded values are no good because the main selection criteria will be the article_id, and then ordered along the lines of the first ‘root level’ article_comment_id for the article followed by its sub-nodes, then the next ‘root level’ article_comment_id and its sub-nodes.

I am providing a sample table and data so you can see what I mean, plus a screen capture showing what I am trying to achieve.

CREATE TABLE test_comment
(
  article_comment_id bigserial NOT NULL,
  article_id bigint NOT NULL,
  parent_comment_id bigint,
  comment text NOT NULL,
  comment_depth integer,
  CONSTRAINT test_comment_pkey PRIMARY KEY (article_comment_id )
)

INSERT INTO test_comment (article_comment_id, article_id, parent_comment_id, comment, comment_depth)
VALUES
 (1, 100, 0, 'First Root Comment', 0)
,(5, 100, 0, 'Second Root Comment', 0)
,(2, 100, 1, 'Reply 1 to Root Comment', 1)
,(3, 100, 2, 'Reply 2 to Reply 1', 2)
,(4, 100, 3, 'Reply 3 to Reply 2', 3)
,(6, 100, 2, 'Reply 4 to Reply 1', 2)
,(7, 100, 5, 'Reply 5 to Second Root Comment', 1);

I have tried this, but it does not provide the proper order:

with recursive comment_list(article_comment_id, parent_comment_id, comment, article_id) AS (
    select c.article_comment_id, c.parent_comment_id, c.comment, c.article_id
    from test_comment c
    where article_id = 100
  union
    select c.article_comment_id, c.parent_comment_id, c.comment, c.article_id
    from test_comment c, comment_list cl
    where c.article_comment_id = cl.article_comment_id
    )
    select * from comment_list;

And can this PostgreSQL specific query be used with JPA, anyway?

Answer 1

You need to “carry” the root article id down to each node and calculate the hierarchy level to be able to get that ordering:

with recursive comment_list(article_comment_id, parent_comment_id, comment, article_id, level, root_id) AS (
    select c.article_comment_id, 
           c.parent_comment_id, 
           c.comment, 
           c.article_id, 
           0 as level,
           article_comment_id as root_id 
    from test_comment c
    where article_id = 100
      and parent_comment_id = 0

  union

    select c.article_comment_id, 
           c.parent_comment_id, 
           c.comment, 
           c.article_id, 
           cl.level + 1,
           cl.article_comment_id 
    from test_comment c
       join comment_list cl on c.parent_comment_id = cl.article_comment_id
)
select article_comment_id, 
     parent_comment_id, 
     comment, 
     article_id
from comment_list
order by root_id, level;

SQLFiddle: http://www.sqlfiddle.com/#!12/af0d6/4

(Btw: you have your join wrong in the recursive part of the union)