Calculate loan payments & when the payment was satisfed - partial payments allowed - mysql

Question

I've been writing SQL queries for years but I'm stuck on this one.
I've got 2 tables in MySQL:
LOANPAYMENTSDUE includes LoanPaymentsDueId, LoanId, AmtDue, DueDate
LOANPAYMENTS includes LoanPaymentsId, LoanId, AmtPaid, PaidDate
The relationship between the tables is the LoanId and not the specific payment that is due. In a perfect world the DueDate = PaidDate and the AmtDue = AmtPaid. However, what is making this complex for me is no relationship between the LoanPaymentsDueId and the LoanPaymentsId. The relationship only exists at the LoanId allowing for partial payments to be made on a single LOANPAYMENTSDUE payment.
I've researched the web trying to find the right query to create a report showing the date that each LOANPAYMENTSDUE was satisfied. This requires calculating the balance as of the LOANPAYMENTSDUE.DueDate because there can be payments missed and a new payment should satisfy the balance of the oldest LOANPAYMENTSDUE payment.
Here is the sample data and table scripts:
CREATE TABLE LOANPAYMENTSDUE (
LoanPaymentsDueId BIGINT(20) NOT NULL AUTO_INCREMENT
, LoanId BIGINT(20)
, AmtDue double NOT NULL
, DueDate date NOT NULL
, PRIMARY KEY (LoanPaymentsDueId)
);
INSERT INTO LOANPAYMENTSDUE (LoanId, AmtDue, DueDate) VALUES (1, 100, '2013-07-15');
INSERT INTO LOANPAYMENTSDUE (LoanId, AmtDue, DueDate) VALUES (1, 100, '2013-08-15');
INSERT INTO LOANPAYMENTSDUE (LoanId, AmtDue, DueDate) VALUES (1, 100, '2013-09-15');
INSERT INTO LOANPAYMENTSDUE (LoanId, AmtDue, DueDate) VALUES (1, 100, '2013-10-15');
INSERT INTO LOANPAYMENTSDUE (LoanId, AmtDue, DueDate) VALUES (1, 100, '2013-11-15');
CREATE TABLE LOANPAYMENTS (
LoanPaymentsId BIGINT(20) NOT NULL AUTO_INCREMENT
, LoanId BIGINT(20)
, AmtPaid double NOT NULL
, PaidDate date NOT NULL
, PRIMARY KEY (LoanPaymentsId)
);
INSERT INTO LOANPAYMENTS (LoanId, AmtPaid, PaidDate) VALUES (1, 100, '2013-07-15'); /* Full pmt on due date */
INSERT INTO LOANPAYMENTS (LoanId, AmtPaid, PaidDate) VALUES (1, 100, '2013-08-10'); /* Full pmt a few days early */
INSERT INTO LOANPAYMENTS (LoanId, AmtPaid, PaidDate) VALUES (1, 100, '2013-09-22'); /* Full pmt a week late */
INSERT INTO LOANPAYMENTS (LoanId, AmtPaid, PaidDate) VALUES (1, 50, '2013-10-18'); /* Partial pmt a few days late */
INSERT INTO LOANPAYMENTS (LoanId, AmtPaid, PaidDate) VALUES (1, 50, '2013-11-07');/* Partial pmt 3 weeks late and satisfies the 10/15/2013 balance on this date */
INSERT INTO LOANPAYMENTS (LoanId, AmtPaid, PaidDate) VALUES (1, 100, '2013-11-22');/* Full pmt a week late and satisfies the 11/15/2013 pmt due */
The report query should simply provide the PAIDDATE when each LOANPAYMENTSDUE was satisfied. Using the table data above the report would be as follows:
LOANID LOANPAYMENTSDUEID AMTDUE DUEDATE PAIDDATE
1 1 100 2013-07-15 2013-07-15
1 2 100 2013-08-15 2013-08-10
1 3 100 2013-09-15 2013-09-22
1 4 100 2013-10-15 2013-11-07
1 5 100 2013-11-15 2013-11-22

Answer 1

You could start with these two queries, that return all of the rows with a running total column:
SELECT
LoanId, DueDate,
CASE WHEN LoanId=#last_LoanId THEN #Due:=#Due+AmtDue
ELSE #Due:=AmtDue END total_due,
#last_LoanId:=LoanId
FROM
LOANPAYMENTSDUE, (SELECT #last_LoanId:=NULL, #Due:=NULL) t;
SELECT
LoanId, PaidDate,
CASE WHEN LoanId=#last_LoanId THEN #Paid:=#Paid+AmtPaid
ELSE #Paid:=AmtPaid END total_paid,
#last_LoanId:=LoanId
FROM
LOANPAYMENTS, (SELECT #last_LoanId:=NULL, #Paid:=NULL) t;
and then you could use a LEFT JOIN on due.LoanId=due.LoanId AND total_due<=total_paid, and a GROUP BY to get the minimum date where the join succeded:
SELECT
ld.LoanId, ld.DueDate, MIN(lp.PaidDate)
FROM
(SELECT
LoanId, DueDate,
CASE WHEN LoanId=#last_LoanId1 THEN #Due:=#Due+AmtDue ELSE #Due:=AmtDue END total_due,
#last_LoanId1:=LoanId
FROM
LOANPAYMENTSDUE, (SELECT #last_LoanId1:=NULL, #Due:=NULL) t1) ld
LEFT JOIN
(SELECT
LoanId, PaidDate,
CASE WHEN LoanId=#last_LoanId2 THEN #Paid:=#Paid+AmtPaid ELSE #Paid:=AmtPaid END total_paid,
#last_LoanId2:=LoanId
FROM
LOANPAYMENTS, (SELECT #last_LoanId2:=NULL, #Paid:=NULL) t2) lp
ON
ld.LoanId=lp.LoanId AND ld.total_due<=lp.total_paid
GROUP BY
ld.LoanId, ld.DueDate
Please see fiddle here.

Answer 2

Assuming that when the amount is paid it's paid in portion or remaining amount in whole, you check based on Total Amount Due and Total Amount Paid by matching those up. Here's the sqlFiddle example of your data and query
SELECT T1.LoanId,
T1.LoanPaymentsDueId,
T1.AmtDue,
T1.DueDate,
T2.PaidDate
FROM
(SELECT
LD.LoanPaymentsDueId,
LD.LoanId,
LD.DueDate,
LD.AmtDue,
(SELECT Sum(AmtDue)
FROM LOANPAYMENTSDUE LD1
WHERE LD1.DueDate <= LD.DueDate
AND LD1.LoanId = LD.LoanId
)as AmtDueTotal
FROM
LOANPAYMENTSDUE LD
)T1,
(SELECT
L.LoanPaymentsId,
L.LoanId,
L.PaidDate,
(SELECT Sum(AmtPaid)
FROM LOANPAYMENTS L1
WHERE L1.PaidDate <= L.PaidDate
AND L1.LoanId = L.LoanId
)as AmtPaidTotal
FROM LOANPAYMENTS L
)T2
WHERE
T1.LoanId = T2.LoanId
AND T1.LoanId = 1
AND T1.AmtDueTotal = T2.AmtPaidTotal;