Custom output from a mysql query - mysql

Question

Consider the following schema:
CREATE TABLE `Result` (
`startDate` date NOT NULL,
`description` varchar(45) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL,
`value` decimal(15,4) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
INSERT INTO `Result`
(`startDate`,
`description`,
`value`)
VALUES
('2020-09-01' ,'Allowance' ,4000),
('2020-09-01' ,'Salary' ,1500),
('2020-10-01' ,'Allowance' ,2000),
('2020-10-01' ,'Salary' ,3000),
('2020-10-01' ,'Deduction' ,-200);
Given a date,the result should show as the total for a description & startdate and the difference between the date selected and previous date(month). So if October was the month selected the result of the query should show as,
description SeptemberTotal OctoberTotal Variance
Allowance 4000 2000 -2000
Salary 1500 3000 1500
Deduction 0 -200 -200
My attempt using a union & a pivot,
SELECT #selectDate:='2020-10-01'; -- set desired date
SELECT
t.month,
t.description,
Gross,
from (
SELECT
DATE_FORMAT(pi.startDate, '%b/%y') AS 'Month',
SUM(pi.value) AS gross,
description
FROM
Result pi
WHERE
pi.startDate = DATE_SUB(#selectDate, INTERVAL 1 MONTH) -- select previous month
GROUP BY description
UNION SELECT
DATE_FORMAT(pi.startDate, '%b/%y') AS 'Month',
SUM(pi.value) AS gross,
description
FROM
Result pi
WHERE
pi.startDate = #selectDate
GROUP BY description) t
GROUP BY t.Month,t.description
;
which gives the result as,
Month description Gross
Sep/20 Allowance 4000
Sep/20 Salary 1500
Oct/20 Allowance 2000
Oct/20 Salary 3000
Oct/20 Deduction -200
which is not exactly what the requirement is. I have tried a pivot query as well, that too is not showing the output as required.
db-fiddle

Answer 1

SET #m1 := '2020-09-01';
SET #m2 := '2020-10-01';
SELECT Result.Description,
COALESCE(SUM(CASE WHEN Result.startDate = #m1 THEN value END), 0) Total1,
COALESCE(SUM(CASE WHEN Result.startDate = #m2 THEN value END), 0) Total2,
COALESCE(SUM(CASE WHEN Result.startDate = #m2 THEN value END), 0) -
COALESCE(SUM(CASE WHEN Result.startDate = #m1 THEN value END), 0) Variance
FROM ( SELECT #m1 startDate UNION ALL SELECT #m2 ) baseDates
LEFT JOIN Result USING (startDate)
GROUP BY Result.Description
fiddle

Answer 2

select
description,
sum(if(startDate between '2020-11-01 00:00:00' and '2020-11-31 23:59:59' ,value,0)) 1st,
sum(if(startDate between '2020-12-01 00:00:00' and '2020-12-31 23:59:59',value,0)) 2nd,
sum(if(startDate between '2020-11-01 00:00:00' and '2020-11-31 23:59:59' ,value,0))-sum(if(created_at between '2020-12-01 00:00:00' and '2020-12-31 23:59:59',value,0)) Variance
from
Result
where startDate between '2020-11-01 00:00:00' and '2020-12-31 23:59:59' group by description