Question

I’m writing sql query to get post and only last comment of this post(if exists).
But I can’t find a way to limit only 1 row for right column in left join.

Here is sample of this query.

SELECT post.id, post.title,comment.id,comment.message
from post
left outer join comment
on post.id=comment.post_id

If post has 3 comments I get 3 rows with this post, but I want only 1 row with last comment(ordered by date).

Can somebody help me with this query?

Answer 1

SELECT  post.id, post.title, comment.id, comment.message
FROM    post
OUTER APPLY
        (
        SELECT  TOP 1 *
        FROM    comment с
        WHERE   c.post_id = post.id
        ORDER BY
                date DESC
        ) comment

or

SELECT  *
FROM    (
        SELECT  post.id, post.title, comment.id, comment.message,
                ROW_NUMBER() OVER (PARTITION BY post.id ORDER BY comment.date DESC) AS rn
        FROM    post
        LEFT JOIN
                comment
        ON      comment.post_id = post.id
        ) q
WHERE   rn = 1

The former is more efficient for few posts with many comments in each; the latter is more efficient for many posts with few comments in each.