Update oldest rows based on sum of column - mysql

Question

We are having a following MySQL table to maintain user credits.
id user credits expiry status
-----------------------------------------
1 A 1.2 somedatetime 0
2 A 4.4 somedatetime 0
3 A 5.0 somedatetime 0
4 B 1.0 somedatetime 0
5 B 2.4 somedatetime 0
6 C 7.8 somedatetime 0
Whenever user makes a purchase, we deduct the amount from the available credits. To be fair to user, the credits with nearest expiry will be consumed first and so on. We will also update the status to mark row as consumed.
For example, if user A makes a purchase of $2, $1.2 will be debited from id 1 and remaining $0.8 from id 2 and so on. So Table will now look like
id user credits expiry status
-----------------------------------------
1 A 0.0 somedatetime 1
2 A 3.6 somedatetime 1
3 A 5.0 somedatetime 0
4 B 1.0 somedatetime 0
5 B 2.4 somedatetime 0
6 C 7.8 somedatetime 0
So far we have been doing it with brute-force approach. Any idea suggestion how to do it more efficiently in minimum or single query.
Update: since someone asked about our current brute-force approach, it's iterating each row from the oldest and updating till the purchase amount is covered, which is very inefficient.
Thanks

Answer 1

Using variables you calculate the totals credits. Run the inner query so you learn what is beign calculate first.
Fiddle Demo
UPDATE customer c
JOIN (
SELECT cu.`id`,
cu.`user`,
`credits`, `expiry`, `status`,
#total := IF(#customer = cu.`user`, #total := #total + `credits`, `credits`) as cumulative_total,
#customer := cu.`user` as user_current,
`credit_used`
FROM customer cu
CROSS JOIN (SELECT #customer := '', #total := 0 ) t
JOIN credits
ON cu.`user` = credits.`user`
ORDER BY cu.`id`
) t
ON c.`id` = t.`id`
SET c.credits = CASE WHEN c.credits <= t.credit_used THEN 0
ELSE t.cumulative_total - credit_used
END,
c.status = CASE WHEN c.credits <= t.credit_used THEN 1
ELSE 0
END;
My test Setup:
CREATE TABLE customer
(`id` int, `user` varchar(1), `credits` double, `expiry` int, `status` int)
;
INSERT INTO customer
(`id`, `user`, `credits`, `expiry`, `status`)
VALUES
(1, 'A', 1.2, 1, 0),
(2, 'A', 4.4, 2, 0),
(3, 'A', 5.0, 3, 0),
(4, 'B', 1.0, 4, 0),
(5, 'B', 2.4, 5, 0),
(6, 'C', 7.8, 6, 0)
;
CREATE TABLE credits
(`id` int, `user` varchar(1), `credit_used` double)
;
INSERT INTO credits
(`id`, `user`, `credit_used`)
VALUES
(1, 'A', 2.0),
(2, 'B', 3.4)
;

Answer 2

http://sqlfiddle.com/#!9/485673/1
SET #amount = 2;
UPDATE t1
JOIN (
SELECT t2.id,
IF(#amount=0,t2.credits, IF(#amount>t2.credits,0,t2.credits-#amount)) credits,
IF(#amount>=t2.credits,#amount := #amount-t2.credits, 0)
FROM (
SELECT id, credits
FROM t1
WHERE credits>0 AND `user`='A'
ORDER BY expiry ASC
) t2
) t
ON t1.id = t.id
SET t1.credits=t.credits
WHERE t1.user = 'A';