Question

I need to combine various MySQL tables for a common result. The needed and expected result is a list of last X comments on a website but the tables are individuals, like: news_comments, pages_comments, etc. Individually, this tables has JOINS to users database.

The structure of 3 of this comments table are:

news_comments, pages_comments, other_comments
id int ai
id_new/id_page/id_other int
id_user int
comment text
created int (timestamp)

The table for users:

id int ai
login int
password varchar
...

I don’t complete this last because we only need id and login.

The normal use of this queries (individuals) are:

SELECT users.id, users.login, news_comments.*, news_comments.id AS id_comment
FROM users, news_comments
WHERE users.id=news_comments.id_user AND news_comments.id_new=$id
ORDER BY created DESC
LIMIT $offset,$rows

And works fine!

But in the backend i want to have a list of all the comments on the website to moderate and control it. I need to combine all the comments tables, order it by created desc and limit the result to make a pagination.

I use MySQL and PHP, so if I need to use PHP there is no problem with that.

How I can do it?

Thanks in advance!

Answer 1

SELECT  *
FROM    (
        SELECT  *
        FROM    (
                SELECT  *
                FROM    news_comments
                WHERE   id_new = $id
                ORDER BY
                        created DESC
                LIMIT $rows
                ) q
        UNION ALL
        SELECT  *
        FROM    (
                SELECT  *
                FROM    pages_comments
                WHERE   id_new = $id
                ORDER BY
                        created DESC
                LIMIT $rows
                ) q
        UNION ALL
        SELECT  *
        FROM    (
                SELECT  *
                FROM    other_comments
                WHERE   id_new = $id
                ORDER BY
                        created DESC
                LIMIT $rows
                ) q
        ) q
JOIN    users u
ON      u.id = q.id_user
ORDER BY
        created DESC
LIMIT   $offset, $rows

Note that limits are applied to each of the inner queries as well so that they could benefit from the indexes.

Create the following indexes:

CREATE INDEX ix_newscomments_new_created ON news_comments (id_new, created)
CREATE INDEX ix_pagescomments_new_created ON pages_comments (id_new, created)
CREATE INDEX ix_othercomments_new_created ON other_comments (id_new, created)

for this to work fast