MYSQL to update values based on where clause and Join - mysql

Question

I am very new to MYSQL, and need to know how to update a table based on average data, and also data in another table.
I have a list of grades out of 10 for pupils
user | score | average grade | Band
-----------------------------------
1 | 4 | 3.5 |
2 | 2 | 2 |
4 | 9 | 9 |
1 | 3 | 3 |
1 | 3.5 | |
2 | 2 | |
I want to update band and a scale of A,B or C to indicate their average score is 0-3, 3,6, or 6-10.
Band A = 0-4
Band B = 4-7
Band C = 7-10
Sometimes there is a delay between a user registering for a test and the score being inputted (as in case of row 5) I want the band to be visible. So this is the final goal result.
user | score | average grade | Band
------------------------------------
1 | 4 | 3.5 | A
2 | 2 | 2 |
4 | 9 | 9 | C
1 | 3 | 3 | A
2 | NULL | 3.5 | A
2 | NULL | 2 |
Also I want the band only to be updated if the user has paid a fee, so I have a seperate table with this data.
User | Paid
-----------
1 | 1
2 | 0
3 | 0
4 | 1
So if a user hasn't paid, then average grade is updated, but Band remains empty (or unchanged if populated)
At present I have a score table and a user table. The user table is a view which calculates the average grade
The only way I can think of doing this without the view, is to have a cron job which runs every 10 minutes that inserts the Bands and average grade into grade table.

Answer 1

Ok firstly there is absolutely no reason why you should store average grade or bands in this table.
You can always calculate it as needed, or if you needed to cache it it could be stored in a separate table that has a single entry per user. As it stands, you are repeating the same information in many records. So actually you don't need an update at all, just drop both of those columns and use a better select statement.
To make things a little bit easier, you could add a band table to keep track of bands.
CREATE TABLE bands (
band varchar(2),
maxval int,
minval int
);
INSERT INTO bands (band,minval,maxval)
VALUES
('A',0,3),
('B',4,6),
('C',7,10);
You should probably index that but it's so small it might not matter. It lets you modify the bands in the future anyway. You could, of course, not use that table and use if statements instead, but I like this way, because then you can do:
SELECT sub.user,sub.average_score,bands.band
FROM (
SELECT u.user, avg(u.score) as average_score
FROM user_tests as u
GROUP BY u.user ) as sub
LEFT JOIN user_paid ON user_paid.user = sub.user
LEFT JOIN bands
ON sub.average_score >= bands.minval
AND sub.average_score <= bands.maxval
AND user_paid.paid = 1
Fiddle
whenever you need average score and band.
Alternatively, if you want to keep those values stored somewhere, add the columns to your user paid table (make it user_info or something) and use this statement (which is really just wrapping the previous statement inside an update)
UPDATE user_info
INNER JOIN (
SELECT sub.user,sub.average_score,bands.band
FROM (
SELECT u.user, avg(u.score) as average_score
FROM user_tests as u
GROUP BY u.user ) as sub
LEFT JOIN user_info ON user_info.user = sub.user
LEFT JOIN bands
ON sub.average_score >= bands.minval
AND sub.average_score <= bands.maxval
AND user_info.paid = 1
) as upselect ON upselect.user = user_info.user
SET user_info.average_score = upselect.average_score,
user_info.band = upselect.band
Fiddle