converting subquery to join in postgresql - sql

Question

select p.postid postid,p.posttypeid posttypeid,
(select count(parentid) as no_of_answers from post
where parentid=p.postid), p.title title,
p.body body ,
p.creationdate creationdate,
p.modifieddate modifieddate,
p.modifieduserid modifieduserid,
p.score score,p.views no_of_views,
u.userid userid
from post
as p
left outer join user_quest as u on p.owneruserid = u.userid
where p.posttypeid = 1
Order by p.creationdate desc
LIMIT 10 OFFSET 0;
I need to convert his subquery to join, please help

Answer 1

You could use CTE:
WITH counts AS (
SELECT
count(parentid) AS no_of_answers,
parentid
FROM post
GROUP BY parentid
)
SELECT
p.postid,
p.posttypeid,
p.title,
COALESCE(c.no_of_answers, 0) AS no_of_answers,
p.body,
p.creationdate,
p.modifieddate,
p.modifieduserid,
p.score,
p.views AS no_of_views,
u.userid
FROM post AS p
LEFT JOIN counts c ON (c.parentid = p.postid)
LEFT JOIN user_quest AS u ON (p.owneruserid = u.userid)
WHERE p.posttypeid = 1
ORDER BY p.creationdate DESC
LIMIT 10 OFFSET 0;
or put yours subquery to JOIN:
SELECT
p.postid,
p.posttypeid,
p.title,
COALESCE(c.no_of_answers, 0) AS no_of_answers,
p.body,
p.creationdate,
p.modifieddate,
p.modifieduserid,
p.score,
p.views AS no_of_views,
u.userid
FROM post AS p
LEFT JOIN
(
SELECT
count(parentid) AS no_of_answers,
parentid
FROM post
GROUP BY parentid
) c ON (c.parentid = p.postid)
LEFT JOIN user_quest AS u ON (p.owneruserid = u.userid)
WHERE p.posttypeid = 1
ORDER BY p.creationdate DESC
LIMIT 10 OFFSET 0;
But I would prefer CTE. The performance of CTEs and subqueries should, in theory, be the same since both provide the same information to the query optimizer. One difference is that a CTE used more than once could be easily identified and calculated once. And it's look pretties because is easier to read, at least i thing so.