具有特定参数的MySQL / PHP平均列值

Question

I have a surprisingly basic question; I thought the answer was going to be more straightforward regarding Averaging columns. Here is my table with sample values:

Table: ratings

id, rating, item_id

Sample data

1, 5, 3
2, 1, 2
3, 2, 3
4, 4, 4 
5, 1, 2

What if I want to get the average rating of the item_id's marked "3"

The real answer would be 2.5 since there is a 3 with a1, and another with a 5.

Here is the query I believe:

 SELECT avg(rating) as average FROM ratings;

How do I specify ONLY the ratings with an item_id of 3? Do I use WHERE? GROUP BY? something else?

And the second part of the question: How do I output this into a single variable?

Answer 1

You simply need to specify a WHERE clause:

SELECT AVG(rating) AS average
FROM ratings
WHERE item_id = 3
-- returns 1 row

If you want to get averages of entire data grouped by item_id you can use GROUP BY:

SELECT item_id, AVG(rating) AS average
FROM ratings
GROUP BY item_id
-- returns as many rows as the number of distinct item_ids