How to find which posts have the highest comments and which posts have the fewest comments? - postgresql

Question

I am very new to postgreSQl and SQL and databases, I hope you guys can help me with this, i want to know which posts have the most amount of comments and which have the least amount of comments and the users need to be specified too.
CREATE SCHEMA perf_demo;
SET search_path TO perf_demo;
-- Tables
CREATE TABLE users(
id SERIAL -- PRIMARY KEY
, email VARCHAR(40) NOT NULL UNIQUE
);
CREATE TABLE posts(
id SERIAL -- PRIMARY KEY
, user_id INTEGER NOT NULL -- REFERENCES users(id)
, title VARCHAR(100) NOT NULL UNIQUE
);
CREATE TABLE comments(
id SERIAL -- PRIMARY KEY
, user_id INTEGER NOT NULL -- REFERENCES users(id)
, post_id INTEGER NOT NULL -- REFERENCES posts(id)
, body VARCHAR(500) NOT NULL
);
-- Generate approx. N users
-- Note: NULL values might lead to lesser rows than N value.
INSERT INTO users(email)
WITH query AS (
SELECT 'user_' || seq || '#'
|| ( CASE (random() * 5)::INT
WHEN 0 THEN 'my'
WHEN 1 THEN 'your'
WHEN 2 THEN 'his'
WHEN 3 THEN 'her'
WHEN 4 THEN 'our'
END )
|| '.mail' AS email
FROM generate_series(1, 5) seq -- Important: Replace N with a useful value
)
SELECT email
FROM query
WHERE email IS NOT NULL;
-- Generate N posts
INSERT INTO posts(user_id, title)
WITH expanded AS (
SELECT random(), seq, u.id AS user_id
FROM generate_series(1, 8) seq, users u -- Important: Replace N with a useful value
),
shuffled AS (
SELECT e.*
FROM expanded e
INNER JOIN (
SELECT ei.seq, min(ei.random) FROM expanded ei GROUP BY ei.seq
) em ON (e.seq = em.seq AND e.random = em.min)
ORDER BY e.seq
)
-- Top 20 programming languages: https://www.tiobe.com/tiobe-index/
SELECT s.user_id,
'Let''s talk about (' || s.seq || ') '
|| ( CASE (random() * 19 + 1)::INT
WHEN 1 THEN 'C'
WHEN 2 THEN 'Python'
WHEN 3 THEN 'Java'
WHEN 4 THEN 'C++'
WHEN 5 THEN 'C#'
WHEN 6 THEN 'Visual Basic'
WHEN 7 THEN 'JavaScript'
WHEN 8 THEN 'Assembly language'
WHEN 9 THEN 'PHP'
WHEN 10 THEN 'SQL'
WHEN 11 THEN 'Ruby'
WHEN 12 THEN 'Classic Visual Basic'
WHEN 13 THEN 'R'
WHEN 14 THEN 'Groovy'
WHEN 15 THEN 'MATLAB'
WHEN 16 THEN 'Go'
WHEN 17 THEN 'Delphi/Object Pascal'
WHEN 18 THEN 'Swift'
WHEN 19 THEN 'Perl'
WHEN 20 THEN 'Fortran'
END ) AS title
FROM shuffled s;
-- Generate N comments
-- Note: The cross-join is a performance killer.
-- Try the SELECT without INSERT with small N values to get an estimation of the execution time.
-- With these values you can extrapolate the execution time for a bigger N value.
INSERT INTO comments(user_id, post_id, body)
WITH expanded AS (
SELECT random(), seq, u.id AS user_id, p.id AS post_id
FROM generate_series(1, 10) seq, users u, posts p -- Important: Replace N with a useful value
),
shuffled AS (
SELECT e.*
FROM expanded e
INNER JOIN ( SELECT ei.seq, min(ei.random) FROM expanded ei GROUP BY ei.seq ) em ON (e.seq = em.seq AND e.random = em.min)
ORDER BY e.seq
)
SELECT s.user_id, s.post_id, 'Here some comment: ' || md5(random()::text) AS body
FROM shuffled s;
Could someone show me how this could be done please, I am new to SQL/postgres any help would be much appreciated. an Example would be very helpful too.

Answer 1

Good effort in pasting the whole dataset creation procedure, is what it needs to be included in order to make the example reproducible.
Let's start first with, how to join several tables: you have your posts table which contains the user_id and we can use it to join with users with the following.
SELECT email,
users.id user_id,
posts.id post_id,
title
from posts join users
on posts.user_id=users.id;
This will list the posts together with the authors. Check the joining condition (after the ON) stating the fields we're using. The result should be similar to the below
email | user_id | post_id | title
------------------+---------+---------+----------------------------------------
user_1#her.mail | 1 | 5 | Let's talk about (5) Visual Basic
user_1#her.mail | 1 | 2 | Let's talk about (2) Assembly language
user_3#her.mail | 3 | 8 | Let's talk about (8) R
user_3#her.mail | 3 | 7 | Let's talk about (7) Perl
user_4#her.mail | 4 | 6 | Let's talk about (6) Visual Basic
user_5#your.mail | 5 | 4 | Let's talk about (4) R
user_5#your.mail | 5 | 3 | Let's talk about (3) C
user_5#your.mail | 5 | 1 | Let's talk about (1) Ruby
(8 rows)
Now it's time to join this result, with the comments table. Since a post can have comments or not and you want to show all posts even if you don't have any comments you should use the LEFT OUTER JOIN (more info about join types here
So let's rewrite the above to include comments
SELECT email,
users.id user_id,
posts.id post_id,
title,
comments.body
from posts
join users
on posts.user_id=users.id
left outer join comments
on posts.id = comments.post_id
;
Check out the join between posts and comments based on post_id.
The result of the query is the list of posts, related author and comments, similar to the below
email | user_id | post_id | title | body
------------------+---------+---------+----------------------------------------+-----------------------------------------------------
user_1#her.mail | 1 | 5 | Let's talk about (5) Visual Basic |
user_1#her.mail | 1 | 2 | Let's talk about (2) Assembly language |
user_3#her.mail | 3 | 8 | Let's talk about (8) R | Here some comment: 200bb07acfbac893aed60e018b47b92b
user_3#her.mail | 3 | 8 | Let's talk about (8) R | Here some comment: 66159adaed11404b1c88ca23b6a689ef
user_3#her.mail | 3 | 8 | Let's talk about (8) R | Here some comment: e5cc1f7c10bb6103053bf281d3cadb60
user_3#her.mail | 3 | 8 | Let's talk about (8) R | Here some comment: 5ae8674c2ef819af0b1a93398efd9418
user_3#her.mail | 3 | 7 | Let's talk about (7) Perl | Here some comment: 5b818da691c1570dcf732ed8f6b718b3
user_3#her.mail | 3 | 7 | Let's talk about (7) Perl | Here some comment: 88a990e9495841f8ed628cdce576a766
user_4#her.mail | 4 | 6 | Let's talk about (6) Visual Basic |
user_5#your.mail | 5 | 4 | Let's talk about (4) R | Here some comment: ed19bb476eb220d6618e224a0ac2910d
user_5#your.mail | 5 | 3 | Let's talk about (3) C | Here some comment: 23cd43836a44aeba47ad212985f210a7
user_5#your.mail | 5 | 1 | Let's talk about (1) Ruby | Here some comment: b83999120bd2bb09d71aa0c6c83a05dd
user_5#your.mail | 5 | 1 | Let's talk about (1) Ruby | Here some comment: b4895f4e0aa0e0106b5d3834af80275e
(13 rows)
Now you can start aggregating and counting comments for a certain post. You can use PG's aggregation functions, we'll use the COUNT here.
SELECT email,
users.id user_id,
posts.id post_id,
title,
count(comments.id) nr_comments
from posts
join users
on posts.user_id=users.id
left outer join comments
on posts.id = comments.post_id
group by email,
users.id,
posts.id,
title
;
Check out that we're counting the comments.id field but we could also perform a count(*) which just counts the rows. Also check that we are grouping our results by email, users.id, post.id and title, the columns we are showing alongside the count.
The result should be similar to
email | user_id | post_id | title | nr_comments
------------------+---------+---------+----------------------------------------+-------------
user_3#her.mail | 3 | 7 | Let's talk about (7) Perl | 2
user_5#your.mail | 5 | 3 | Let's talk about (3) C | 1
user_5#your.mail | 5 | 1 | Let's talk about (1) Ruby | 2
user_3#her.mail | 3 | 8 | Let's talk about (8) R | 4
user_1#her.mail | 1 | 5 | Let's talk about (5) Visual Basic | 0
user_5#your.mail | 5 | 4 | Let's talk about (4) R | 1
user_4#her.mail | 4 | 6 | Let's talk about (6) Visual Basic | 0
user_1#her.mail | 1 | 2 | Let's talk about (2) Assembly language | 0
(8 rows)
This should be the result you're looking for. Just bear in mind, that you're showing the user from users who wrote the post, not the one who commented. To view who commented you'll need to change the joining conditions.