User recent activities - PHP MySQL

Question

I have 3 MySQL tables representing: photos a user post, videos a user post, comments a user post and I need to view the 10 (20, 30, 40...) most recent activities by the user.

For example in photos table may be composed by:

user_id | photo_id   | photo_path | photo_name | date_added
   5    |     18     |  /photos   | pht_18.png |  2009-02-12
   5    |     21     |  /photos   | pht_21.png |  2009-02-15
   5    |     29     |  /photos   | pht_29.png |  2009-03-30 

the videos table

user_id | video_id   |  video_url       |   date_added
   5    |     36     |  youtube.com/... |   2009-01-09
   5    |     48     |  youtube.com/... |   2009-02-18
   5    |     90     |  youtube.com/... |   2009-03-19

the comments table

user_id | comment_id |   comment     |  date_added
   5    |     6      |   hi!         |  2009-02-11
   5    |     11     |   great photo |  2009-02-13
   5    |     19     |   nice shot!  |  2009-03-28

As you can see the 3 tables have different number of attributes, so how can I do the union? and while fetching the query result how can I understand to which table it belongs to?

So in the user profile page I'd like to show his recent activities of course ordered by DATE DESC this way:

2009-09-01: user posted a video
2009-11-02: user posted a comment
2009-12-02: user posted a photo
2009-13-02: user posted a comment
2009-15-02: user posted a photo
2009-18-02: user posted a video
2009-19-03: user posted a video
2009-28-03: user posted a comment
2009-30-03: user posted a photo

Can anyone help me please?

Answer 1

A MySQL UNION query could work here:

(SELECT `user_id`, `date_added`, 'photo' AS `type` FROM `photos` WHERE `user_id` = uid) UNION
 (SELECT `user_id`, `date_added`, 'video' AS `type` FROM `videos` WHERE `user_id` = uid) UNION
 (SELECT `user_id`, `date_added`, 'comment' AS `type` FROM `comments` WHERE `user_id` = uid)
 ORDER BY `date_added` DESC;

Then you'd wind up with a result set like

user_id | date_added | type
    5   | 2009-01-03 | photo
    5   | 2008-12-07 | video
    5   | 2008-11-19 | comment

and so on. (actually you can leave user_id out of the SELECT if you want, of course)

Answer 2

Personally, I would make another table to store any activity. It would simplify things a lot and you could also keep track of deletions and other activities.

Answer 3

Why do you have separate tables in the first place? That's probably a mistake in database design.

[EDIT: As it turned out through comments and a question edit, the OP had a valid reason to maintain three tables. Further advice about this removed.]

To solve your problem you can use UNION or UNION ALL:

(SELECT 'photo'   AS item_type, date_added, user_id FROM photos)
UNION ALL
(SELECT 'video'   AS item_type, date_added, user_id FROM videos)
UNION ALL
(SELECT 'comment' AS item_type, date_added, user_id FROM comments)
ORDER BY date_added DESC