Comments and replies with Postgres - sql

Question

I have the SQL database tables posts, comments, and replies
posts
postid
body
created_at
1
The bucks beat the bills
1/16
2
Soccer tricks and tips
1/17
comments
commentid
postid(references posts.postid)
body
created_at
78
1
Yayyy
1/18
79
1
Booo
1/19
79
2
These tips suck
1/20
replies
replyid
commentid(references comments.commentid)
body
created_at
167
79
I agree
1/21
167
78
yayyyy
1/22
168
79
No they dont
1/23
I want to do 2 things
1
We are GIVEN the postid. For instance, postid=1.
In a single call to the database, I want to query the database to GET:
the post body
the first 2 comments on a post, sorted by created_at
for each of those comments, get the first 2 replies, sorted by created_at
Querying POST, COMMENTS, and REPLIES from the database using SQL would look something like,
const POST =
fetch(
select * from posts
where postid = 1
)
const COMMENTS =
fetch(
select * from comments
where postid = ${POST.postid}
order by created_at
limit 2
)
const REPLIES =
COMMENTS.map((COMMENT) => {
fetch(
select * from replies
where commentid = ${COMMENT.commentid}
order by created_at
limit 2
)
})
How do I write these queries as a SINGLE SQL call to the database?
The returned data should be in a nested form. Something like
const POST = {
postid: 1
body: "...",
comments: [{commentid: 1, body: "..."}, ...]
}
But if you have a different form that is easier, I'm open.
2
In the call above, how do I include the comments and replies aggregates?
For instance, the returned data should look like
const POST = {
// same as before
postid: 1
body: "...",
comments: [{commentid: 1, body: "..."}, ...]
// aggregate for comments
comments_aggregate: 2
}

Answer 1

Your schema is incorrect. You cannot reference column commentid if it is not unique. Assuming you properly make the postid, commentid, replyid the respective PRIMARY KEY of their tables, this method should provide the information you are after.
Few comments inline.
SELECT json_build_object(
'postid', p.postid,
'body', p.body,
'comments', json_agg(
json_build_object(
'commentid', c.commentid,
'body', c.body,
'replies', replies,
'reply_aggregate', replycount
)
),
'comments_aggregate', CommentCount
)
FROM posts p
LEFT OUTER JOIN (
SELECT postid, commentid, body, created_at, CommentCount
FROM (
SELECT *,
/* Count the total number of comments per post */
COUNT(*) OVER (PARTITION BY postid) AS CommentCount,
/* Number the comments of a post to later select the first 2 */
ROW_NUMBER() OVER (PARTITION BY postid ORDER BY created_at) as CommentNumber
FROM Comments
) FirstTwoComments
WHERE CommentNumber <= 2
) c ON p.postid = c.postid
LEFT OUTER JOIN (
SELECT commentid,
ReplyCount,
json_agg(
json_build_object(
'replyid', replyid,
'body', body
)
) AS replies
FROM (
SELECT *,
/* Count the total number of replies per comment */
COUNT(*) OVER (PARTITION BY commentid) AS ReplyCount,
/* Number the replies of a comment to later select the first 2 */
ROW_NUMBER() OVER (PARTITION BY commentid ORDER BY created_at) as CommentNumber
FROM Replies
) FirstTwoReplies
WHERE CommentNumber <= 2
GROUP BY commentid, ReplyCount
) r ON c.commentid = r.commentid
WHERE p.postid = 1
GROUP BY p.postid, p.body, CommentCount