How to select the last 2 entries from another related table

Question

I have query that selects all users based on their sum of donation from another table and orders it by the sum of their donations.

But I would also like to select each user's last 2 comments where comment type is donation_comment by joining the them together with a space. And to be also able to search by user comments. If I specify where comment_text contains 'comment three' then only 1 entry would show for Sergey Brin.

I can't seem to figure out how to pull in their last comments and add where condition based on it.

So the result would be this

Array
    (
        [0] => stdClass Object
            (
                [id] => 2
                [username] => Sergey Brin
                [donation] => 500
                [last_comments] => comment four comment three
            )

        [1] => stdClass Object
            (
                [id] => 1
                [username] => Larry Page
                [donation] => 400
                [last_comments] => comment five comment two
            )
    )

Here's my current query

SELECT
    users.id,
    users.username,
    sum(donations.donation) as donation
from
    users
inner join donations
    on users.id = donations.user_id
where
    users.username like '%r%'
group by
    users.id,
    users.username
having
    sum(donations.donation) >= 400
order by
    donation desc

users table

id |   username   |
1     Larry Page
2     Sergey Brin

donations table

id | user_id | donation |     date      |
1      1         100       2019-02-12
2      1         200       2019-02-13
3      2         500       2019-01-15
4      1         100       2019-04-10

user_comments table

id | user_id |   comment_text   |        type        |
1       1        comment one       donation_comment
2       1        comment two       donation_comment
3       2        comment three     donation_comment
4       2        comment four      donation_comment 
5       1        comment five      donation_comment

Answer 1

I would make a subquery from your user_comments table where you limit the number of comments per user_id to 2. Then you can use string_agg() to concatinate the comments

Try this:

SELECT
    users.id,
    users.username,
    sum(donations.donation) as donation,
    string_agg(comment_text, ', ') as comments
from
    users
inner join donations
    on users.id = donations.user_id
inner join (
    SELECT* from user_comments
    group by user_id
    limit 2
    ) as last2_don on users.id = last2_don.user_id
where
    users.username like '%r%'
group by
    users.id,
    users.username
having
    sum(donations.donation) >= 400
order by
    donation desc

Answer 2

A lateral join is a very reasonable approach:

select u.id, u.username,
       sum(d.donation) as donation,
       uc.comments_2
from users u inner join
     donations d
     on u.id = d.user_id left join lateral
     (select string_agg(comment_text, '; ') as comments_2
      from (select uc.*
            from user_comments uc
            where uc.user_id = u.id and
                  uc.type = 'donation_comment';
            order by uc.id desc
            limit 2
           ) c
      ) uc
      on 1=1
where u.username like '%r%'
group by u.id, u.username, uc.comments
having sum(d.donation) >= 400
order by sum(donation) desc;