MySQL: Count in other tables - mysql

Question

in my MySQL database I have three tables:
CREATE TABLE favorites (
id int(11) NOT NULL AUTO_INCREMENT,
user_id int(11) NOT NULL,
location_id int(11) NOT NULL,
PRIMARY KEY (id)
);
CREATE TABLE locations (
id int(20) NOT NULL,
`name` varchar(150) NOT NULL,
pos_lat float NOT NULL,
pos_lon float NOT NULL,
PRIMARY KEY (id)
);
CREATE TABLE ratings (
id int(11) NOT NULL AUTO_INCREMENT,
location_id int(11) NOT NULL,
user_id int(11) NOT NULL
stars int(11) NOT NULL,
review text,
PRIMARY KEY (id)
);
Now I want to select some of the locations and calculate the number of ratings, the average number of stars and the number of favorites in an efficient way.
My approach is this one but it gives me totally wrong values for the COUNTs.
SELECT l.id AS location_id,
COUNT(DISTINCT r.id), AVG(r.stars), COUNT(DISTINCT f.id)
FROM locations l, ratings r, favorites f
WHERE (l.id=r.location_id OR l.id=f.location_id)
AND l.id IN (7960,23713,...,18045,24247)
GROUP BY l.id
Can you help me?

Answer 1

The problem has to do with your join condition using OR:
WHERE (l.id=r.location_id OR l.id=f.location_id)
When it finds ONE record where l.id = r.location_id it will be true for ALL rows in f because of the OR. Similarly when it finds 1 record with l.id = f.location_id you will match ALL rows in r.
Instead, use a LEFT JOIN for each of r and f:
SELECT l.id AS location_id,
COUNT(DISTINCT r.id), AVG(r.stars), COUNT(DISTINCT f.id)
FROM locations l
LEFT JOIN ratings r ON (l.id = r.location_id)
LEFT JOIN favorites f ON (l.id = f.location_id)
WHERE l.id IN (7960,23713,...,18045,24247)
GROUP BY l.id