SUM() не работи в MySQL:SUM() с DISTINCT

Поставяте DISTINCT IN sum( rating.rating) as total_rating, затова резултатът (12=17-5 ), тъй като ще включва 5 само веднъж при изчисляване на сумата.

 select review.comments, review.user_id, count(distinct rating.id) as rating_count,
    sum( rating.rating) as total_rating from users 
    left join review on users.id = review.user_id and review.shop_id='1' 
    left join rating on users.id = rating.user_id and rating.shop_id='1' 
    where review.shop_id='1' or rating.shop_id='1' 
    group by users.id, review.user_id, rating.user_id, review.id

Ето SQLPiddle

Примерен изход: Надявам се това да помогне