MySQL：GROUP_CONCAT，AVG和COUNT问题

Question

I have a mySQL statement that looks like below. It tries to select all restaurants (1 per row), concat the cuisines for each restaurant (as there may be 1 or more cuisines per restaurant) into a column for each restaurant, and likewise for the average rating and number of ratings.

I have setup 3 test restaurants. Restaurant 1 has 2 cuisine types and 3 ratings. The problem is that the number of ratings returns double the actual value and the cuisines return as (cuisine1 3 times then cuisine2 3 times) eg (cuisine1, cuisine1, cuisine1, cuisine2, cuisine2, cuisine2). Do you know what could be causing this? Thanks for any help.

SELECT
  r.*,
  GROUP_CONCAT(c.cuisine SEPARATOR ', ') cuisine,
  ROUND(AVG(re.avg_rating),1) rating,
  COUNT(re.restaurant_id) num_ratings
FROM eat_eat_restaurants r
JOIN eat_eat_restaurant_cuisines_lookup rc ON (r.restaurant_id=rc.restaurant_id)
JOIN eat_eat_cuisines c ON (rc.cuisine_id=c.cuisine_id)
LEFT JOIN eat_eat_reviews re ON (r.restaurant_id=re.restaurant_id)
WHERE r.published=1
GROUP BY r.restaurant_id

Answer 1

GROUP BY groups the results, not the tables individually.

In your case you want

1. The restaurant info.
2. The cuisines.
3. The ratings.

Since the "cuisines" and the "ratings" are not dependent with each other, one of them need to be grouped by itself:

SELECT
  r.*,
  GROUP_CONCAT(c.cuisine SEPARATOR ', ') cuisine,
  reg.rating,
  reg.num_ratings
FROM eat_eat_restaurants r
    JOIN eat_eat_restaurant_cuisines_lookup rc ON r.restaurant_id=rc.restaurant_id
    JOIN eat_eat_cuisines c ON rc.cuisine_id=c.cuisine_id
    LEFT JOIN (
        SELECT re.restaurant_id,
            ROUND(AVG(re.avg_rating),1) rating,
            COUNT(re.restaurant_id) num_ratings
        FROM eat_eat_reviews re
        GROUP BY re.restaurant_id
    ) reg ON r.restaurant_id=reg.restaurant_id
WHERE r.published=1
GROUP BY r.restaurant_id