（My）SQL左/右/内部联接的方向

Question

While getting a list of restaurant I want to join rating data in this list.

For example

Table1 – restaurant
Columns: id, name, latitude, longitude

and

Table2 – ratings
Columns: id, restaurant_id, rating (value options: 1, 2, 3, 4, 5)

Constructed this:

SELECT *, SUM(ratings.rating) AS total_rating_count 
FROM restaurants 
INNER JOIN ratings 
    ON ratings.restaurant_id = restaurants.id

This only returns me 1 restaurant. I also need the amount of ratings available per restaurant in able to calculate average rating.

Can use some direction on this.

EDIT : And while we are at it, I want to join another table with comments linked to restaurant. I should dive into this at the same time: What is the difference between Left, Right, Outer and Inner Joins?

Answer 1

You need GROUP BY clause:

SELECT 
  *, AVG(ratings.rating) AS avg_rating_count 
FROM 
  restaurants 
    INNER JOIN ratings ON ratings.restaurant_id = restaurants.id 
GROUP BY 
  restaurants.id

-since you've mentioned average value, I've placed AVG() function in query. But be aware that this is invalid in normal SQL (mixing of non-group columns with group functions) - and in terms of MySQL DBMS doesn't specify which row will be returned.