Update the next row of the target row in MySQL - mysql

Question

Suppose I have a table that tracks if a payment is missed like this:
+----+---------+------------+------------+---------+--------+
| id | loan_id | amount_due | due_at | paid_at | missed |
+----+---------+------------+------------+---------+--------+
| 1 | 1 | 100 | 2013-08-17 | NULL | NULL |
| 5 | 1 | 100 | 2013-09-17 | NULL | NULL |
| 7 | 1 | 100 | 2013-10-17 | NULL | NULL |
+----+---------+------------+------------+---------+--------+
And, for example, I ran a query that checks if a payment is missed like this:
UPDATE loan_payments
SET missed = 1
WHERE DATEDIFF(NOW(), due_at) >= 10
AND paid_at IS NULL
Then suppose that the row with id = 1 gets affected. I want the amount_due of row with id = 1 be added to the amount_due of the next row so the table would look like this:
+----+---------+------------+------------+---------+--------+
| id | loan_id | amount_due | due_at | paid_at | missed |
+----+---------+------------+------------+---------+--------+
| 1 | 1 | 100 | 2013-08-17 | NULL | 1 |
| 5 | 1 | 200 | 2013-09-17 | NULL | NULL |
| 7 | 1 | 100 | 2013-10-17 | NULL | NULL |
+----+---------+------------+------------+---------+--------+
Any advice on how to do it?
Thanks

Answer 1

Take a look at this :
SQL Fiddle
MySQL 5.5.32 Schema Setup:
CREATE TABLE loan_payments
(`id` int, `loan_id` int, `amount_due` int,
`due_at` varchar(10), `paid_at` varchar(4), `missed` varchar(4))
;
INSERT INTO loan_payments
(`id`, `loan_id`, `amount_due`, `due_at`, `paid_at`, `missed`)
VALUES
(1, 1, 100, '2013-09-17', NULL, NULL),
(3, 2, 100, '2013-09-17', NULL, NULL),
(5, 1, 100, '2013-10-17', NULL, NULL),
(7, 1, 100, '2013-11-17', NULL, NULL)
;
UPDATE loan_payments AS l
LEFT OUTER JOIN (SELECT loan_id, MIN(ID) AS ID
FROM loan_payments
WHERE DATEDIFF(NOW(), due_at) < 0
GROUP BY loan_id) AS l2 ON l.loan_id = l2.loan_id
LEFT OUTER JOIN loan_payments AS l3 ON l2.id = l3.id
SET l.missed = 1, l3.amount_due = l3.amount_due + l.amount_due
WHERE DATEDIFF(NOW(), l.due_at) >= 10
AND l.paid_at IS NULL
;
Query 1:
SELECT *
FROM loan_payments
Results:
| ID | LOAN_ID | AMOUNT_DUE | DUE_AT | PAID_AT | MISSED |
|----|---------|------------|------------|---------|--------|
| 1 | 1 | 100 | 2013-09-17 | (null) | 1 |
| 3 | 2 | 100 | 2013-09-17 | (null) | 1 |
| 5 | 1 | 200 | 2013-10-17 | (null) | (null) |
| 7 | 1 | 100 | 2013-11-17 | (null) | (null) |

Answer 2

Unfortunately I don't have time at the moment to write out full-blown SQL, but here's the psuedocode I think you need to implement:
select all DISTINCT loan_id from table loan_payments
for each loan_id:
set missed = 1 for all outstanding payments for loan_id (as determined by date)
select the sum of all outstanding payments for loan_id
add this sum to the amount_due for the loan's next due date after today
Refer to this for how to loop using pure MySQL: http://dev.mysql.com/doc/refman/5.7/en/cursors.html

Answer 3

I fixed my own problem by adding a missed_at field. I put the current timestamp ($now) in a variable before I update the first row to missed = 1 and missed_at = $now then I ran this query to update the next row's amount_due:
UPDATE loan_payments lp1 JOIN loan_payments lp2 ON lp1.due_at > lp2.due_at
SET lp1.amount_due = lp2.amount_due + lp1.amount_due
WHERE lp2.missed_at = $now AND DATEDIFF(lp1.due_at, lp2.due_at) <= DAYOFMONTH(LAST_DAY(lp1.due_at))
I wish I could use just use LIMIT 1 to that query but it turns out that it's not possible for an UPDATE query with a JOIN.
So all in all, I used two queries to achieve what I want. It did the trick.
Please advise if you have better solutions.
Thanks!