Question

I have two tables Posts and comments

Post Table

Post_id
Post_content

Comments table

comment_id
Comment
post_id
created_date

One post can have multiple comments or zero comments

My requirement is getting the latest comment for the posts using left outer join .

I mean result should be one record for the post with the below columns .

post_id,post_content ,comment_id,comment 

In simple words posts should be getting along with their latest comment if it exists.

( Currently the system is getting the posts first and then going to the server again to get the latest comments to display , thought of getting them in one shot since we are displaying only one comment initially … Not sure what should be the best approach if wants to display more than one comment ..?)

Thank You

Regards

Kiran

Answer 1

SELECT Post.post_id, post_content, comment_id, comment
FROM
    Post LEFT JOIN Comments
        ON Post.post_id = Comments.post_id
        AND created_date = (
            SELECT MAX(created_date)
            FROM Comments
            WHERE Post.post_id = Comments.post_id
        )

BTW, you should consider indexing Comments {post_id, created_date} for optimal performance, but be mindful of the non-primary key index overhead in case you are using InnoDB (see the “Disadvantages of clustering” section in this article).