How to get multiple counts from multiple tables? - php

Question

On a webpage, I am displaying a number of picture collections (I show the thumbnails for each collection). Each picture has five relevant tables:
likes (id, user_id, picture_id),
views (id, user_id, picture_id),
comments (id, user_id, picture_id, comment),
pictures (id (which equals the "picture_id" in the previous tables), collection_id, picture_url and several other columns),
collections (id (equal to collection_id in previous table), and several other columns.
When loading my page, I need to aggregate the number of likes, views and comments for all pictures in each collection, so as to show those numbers under each collection.
So basically: count the likes for each picture, count them all up, display number. Count the views for each picture, count them all up, display number. Count the comments for each picture, count them all up, display number. And then rinse and repeat for all collections.
I'm pretty new at mysql, and I'm struggling between selects, multiple joins, counts, php vs mysql, etc etc. I'm sure there's many ways I can do this that would be very inefficient, so I'm hoping you can tell me the best/fastest/most efficient way to do this.
Thanks in advance!

Answer 1

You can solve this with selects and left joins.
Since you'll count entries on each table for every pictureId, your pictures table will be the left side of each relation. So:
select
p.id as pictureId,
count(distinct l.id) as count_likes,
count(distinct v.id) as count_views,
count(distinct c.id) as count_comments
from
pictures as p
left join likes as l on p.id = l.pictureId
left join views as v on p.id = v.pictureId
left join comments as c on p.id = c.pictureId
group by
p.id
Basically, you are counting every record in each table for each record in the pictures table; if there are no records in likes, views or comments, the count will be zero, respectively.
Of course, you can expand this idea for collections:
select
c.id as collection_id,
p.id as picture_id,
count(distinct l.id) as count_likes,
count(distinct v.id) as count_views,
count(distinct c.id) as count_comments
from
collections as c
left join pictures as p on c.id = p.collection_id
left join likes as l on p.id = l.picture_Id
left join views as v on p.id = v.picture_Id
left join comments as c on p.id = c.picture_Id
group by
c.id,
p.id
If you want to filter your results for each collection, you only need to add where c.id = aValue before the group by (where aValue is the collection Id you want to retrieve)
Hope this helps you.
If you only need the aggregate data for each collection:
select
c.id as collection_id,
count(distinct l.id) as count_likes,
count(distinct v.id) as count_views,
count(distinct c.id) as count_comments
from
collections as c
left join pictures as p on c.id = p.collection_id
left join likes as l on p.id = l.picture_Id
left join views as v on p.id = v.picture_Id
left join comments as c on p.id = c.picture_Id
group by
c.id
This should do the trick ;-)

Answer 2

You could do this with subselects:
SELECT
collections.*,
( SELECT COUNT(*) FROM pictures, likes
WHERE pictures.id = likes.picture_id
AND pictures.collection_id = collection.id
) AS like_count,
( SELECT COUNT(*) FROM pictures, views
WHERE pictures.id = views.picture_id
AND pictures.collection_id = collection.id
) AS view_count,
( SELECT COUNT(*) FROM pictures, comments
WHERE pictures.id = comments.picture_id
AND pictures.collection_id = collection.id
) AS comment_count
FROM collections
WHERE ...
This looks like it's going over the pictures table thrice, but I suspect that MySQL might be able to optimize that using the join buffer. I should note that I haven't actually tested this query, however. I also have no idea how this compares performance-wise with Barranka's LEFT JOIN solution. (Both would be pretty horrible if implemented naïvely, so it comes down to how smart MySQL's query optimizer is in each case.)