Question

How can I use MySQL to count with a LEFT JOIN?

I have two tables, sometimes the Ratings table does not have ratings for a photo so I thought LEFT JOIN is needed but I also have a COUNT statement..

Photos

id  name        src
1   car         bmw.jpg
2   bike        baracuda.jpg

Loves (picid is foreign key with photos id)

id  picid   ratersip
4   1       81.0.0.0
6   1       84.0.0.0
7   2       81.0.0.0

Here the user can only rate one image with their IP.

I want to combine the two tables in order of the highest rating. New table

Combined

id  name    src            picid
1   car     bmw.jpg        1
2   bike    baracuda.jpg   2

(bmw is highest rated)

My MySQL code:

SELECT * FROM photos 
LEFT JOIN ON photos.id=loves.picid 
ORDER BY COUNT (picid);

My PHP Code: (UPDATED AND ADDED – Working Example…)

$sqlcount = "SELECT p . *
FROM `pics` p
LEFT JOIN (

SELECT `loves`.`picid`, count( 1 ) AS piccount
FROM `loves`
GROUP BY `loves`.`picid`
)l ON p.`id` = l.`picid`
ORDER BY coalesce( l.piccount, 0 ) DESC";

$pics = mysql_query($sqlcount);

Answer 1

MySQL allows you to group by just the id column:

select
    p.*
from
    photos p 
    left join loves l on
        p.id = l.picid
group by
    p.id
order by
    count(l.picid)

That being said, I know MySQL is really bad at group by, so you can try putting the loves count in a subquery in your join to optimize it:

select
    p.*
from
    photos p
    left join (select picid, count(1) as piccount from loves group by picid) l on
        p.id = l.picid
order by
    coalesce(l.piccount, 0)

I don’t have a MySQL instance to test out which is faster, so test them both.