Speed up the following queries by merging them into one - mysql

Question

I need to find a way for merging two queries into one.
This is the structure of the tables I am using:
(there are also other fields on contents and ratings but I didn't add them, since they aren't needed for this.
-- Create syntax for TABLE 'contents'
CREATE TABLE `contents` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`rating` decimal(5,4) DEFAULT '0.0000',
`ratingsCount` int(8) DEFAULT '0',
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8;
-- Create syntax for TABLE 'ratings'
CREATE TABLE `ratings` (
`what` int(11) DEFAULT NULL,
`time` timestamp NULL DEFAULT CURRENT_TIMESTAMP,
`rating` decimal(3,2) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
Since last time I asked something here on stack overflow someone told me to write the code I'm using right now. Here it is:
db.query("SELECT AVG(rating) `avg`, COUNT(rating) cnt FROM `ratings` WHERE what = ?", [req.params.id], function(err, avg) {
db.query("UPDATE contents SET `rating` = ?, `ratingsCount` = ? WHERE id = ?", [avg[0].avg, avg[0].cnt, req.params.id], function() { });
});

Answer 1

You could use an UPDATE/JOIN combination to do it in a single round trip to the database;
UPDATE contents c
JOIN (
SELECT what, AVG(rating) rating, COUNT(rating) ratingsCount
FROM ratings WHERE what = ? GROUP BY what
) r
ON c.id = r.what
SET c.rating = r.rating, c.ratingsCount = r.ratingsCount
An SQLfiddle to test with.
The subquery will find the average/count for the value of "what", the outer query will just join that information to update contents.

Answer 2

This one will do the work:
UPDATE contents c SET
rating=(SELECT AVG(rating) FROM ratings r WHERE r.what=c.id),
ratingsCount=(SELECT COUNT(rating) FROM ratings r WHERE r.what=c.id);
You'll need to add index for id and what columns.