MYSQL Cumulative Totals with SUM and Subquery - mysql

Question

I have the below table:
CREATE TABLE `_loans` (
`loan_id` int(11) NOT NULL,
`price` float(7,2) NOT NULL,
`term` int(11) NOT NULL,
`app_date` date NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
INSERT INTO `_loans` (`loan_id`, `price`, `term`, `app_date`) VALUES
(1, 299.00, 24, '2019-01-23'),
(2, 774.00, 24, '2019-01-24'),
(3, 817.80, 24, '2019-01-24'),
(4, 279.99, 24, '2019-01-28'),
(5, 463.99, 24, '2019-01-28'),
(6, 0.00, 24, '2019-01-28'),
(7, 357.00, 24, '2019-02-02'),
(8, 386.98, 24, '2019-02-04'),
(9, 846.00, 24, '2019-02-04'),
(10, 713.99, 24, '2019-02-06'),
(11, 0.00, 24, '2019-02-07'),
(12, 579.00, 24, '2019-02-11'),
(13, 179.00, 24, '2019-02-13'),
(14, 0.00, 24, '2019-02-19'),
(15, 259.00, 24, '2019-02-21'),
(16, 249.99, 24, '2019-02-26'),
(17, 319.00, 24, '2019-03-02'),
(18, 1108.99, 24, '2019-03-05'),
(19, 319.00, 24, '2019-03-05'),
(20, 199.97, 24, '2019-03-06');
ALTER TABLE `_loans`
ADD PRIMARY KEY (`loan_id`),
ADD KEY `app_date` (`app_date`);
And the below query, which gives a monthly summary of the table data:
SELECT
MONTHNAME(w.app_date) month,
YEAR(w.app_date) year,
COUNT(*) contracts,
SUM(w.total_price) totals,
#running_total := #running_total + SUM(w.total_price) running_totals
FROM (
SELECT
app_date,
SUM(price * term) total_price
FROM _loans l
GROUP BY l.loan_id
) w
JOIN (SELECT #running_total := 0) r
GROUP BY YEAR(w.app_date), MONTH(w.app_date)
ORDER BY YEAR(w.app_date), MONTH(w.app_date) ASC
I need a running total of the values from the 'totals' column. Everything works well except the cumulative total, which is not accumulating.
month | year | contracts | totals | running_totals
------------------------------------------------------------------
January | 2019 | 6 | 63234.72 | 63234.71923828125
February | 2019 | 10 | 85703.04 | 85703.04016113281
March | 2019 | 4 | 46727.04 | 46727.039794921875

Answer 1

This isn't working because of the GROUPing. You need to nest the GROUPing as a subquery and compute the running total from that:
SELECT monthname as month, year, contracts, totals,
#running_total := #running_total + totals AS running_totals
FROM (SELECT
MONTHNAME(w.app_date) monthname,
MONTH(w.app_date) monthnum,
YEAR(w.app_date) year,
COUNT(*) contracts,
SUM(w.total_price) totals
FROM (SELECT
app_date,
SUM(price * term) total_price
FROM _loans l
GROUP BY l.loan_id
) w
GROUP BY year, monthnum, monthname
) t
JOIN (SELECT #running_total := 0) r
ORDER BY year, monthnum ASC
Output:
month year contracts totals running_totals
January 2019 6 63234.72 63234.71923828125
February 2019 10 85703.04 148937.75939941406
March 2019 4 46727.04 195664.79919433594
Demo on dbfiddle

Answer 2

Your query is overly complicated. The subquery is not necessary for the aggregation. However, it is necessary for the use of variables:
SELECT month, year, total_price,
(#running_total := #running_total + total_price) as running_totals
FROM (SELECT MONTHNAME(l.app_date) as month,
YEAR(l.app_date) as year,
MONTH(l.app_date) as mon,
COUNT(DISTINCT l.loan_id) as contracts,
SUM(l.price * l.term) total_price
FROM _loans l
GROUP BY l.loan_id, YEAR(w.app_date), MONTH(w.app_date)
ORDER BY l.loan_id, YEAR(w.app_date), MONTH(w.app_date)
) l CROSS JOIN
(SELECT #running_total := 0) params
ORDER BY year, mon ASC