MariaDB 10.3

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CREATE TABLE sales (
    id int auto_increment primary key,
    customerID VARCHAR(255),
    order_date DATE,
    sales_volume INT,
    returned_volume INT
);

INSERT INTO sales
(customerID, order_date, sales_volume, returned_volume
)
VALUES 
("Customer_01", "2020-01-01", "200", "50"),
("Customer_01", "2020-02-15", "400", "120"),
("Customer_01", "2020-02-21", "300", "0"),
("Customer_01", "2020-03-17", "800", "480"),
("Customer_01", "2020-04-08", "600", "315"),
("Customer_02", "2020-02-02", "500", "175"),
("Customer_02", "2020-02-03", "900", "620"),
("Customer_02", "2020-03-27", "100", "30"),
("Customer_02", "2020-04-15", "375", "120"),
("Customer_03", "2020-08-12", "700", "280"),
("Customer_04", "2020-03-09", "260", "110");


SELECT
customerID,
SUM(sales_volume),
SUM(returned_volume),
(SUM(returned_volume) / SUM(sales_volume)) AS return_rate
FROM sales
GROUP BY 1
LIMIT 3;


customerID
SUM(sales_volume)
SUM(returned_volume)
return_rate


Customer_01
2300
965
0.4196

Customer_02
1875
945
0.5040

Customer_03
700
280
0.4000


SELECT customerID, SUM(sales_volume), SUM(returned_volume),
       (SUM(returned_volume) / SUM(sales_volume)) AS return_rate
FROM (SELECT s.*,
             ROW_NUMBER() OVER (PARTITION BY customerID ORDER BY order_date DESC) as seqnum
      FROM sales s
     ) s
WHERE seqnum <= 3
GROUP BY 1


customerID
SUM(sales_volume)
SUM(returned_volume)
return_rate


Customer_01
1700
795
0.4676

Customer_02
1375
770
0.5600

Customer_03
700
280
0.4000

Customer_04
260
110
0.4231

customerID	SUM(sales_volume)	SUM(returned_volume)	return_rate
Customer_01	2300	965	0.4196
Customer_02	1875	945	0.5040
Customer_03	700	280	0.4000

customerID	SUM(sales_volume)	SUM(returned_volume)	return_rate
Customer_01	1700	795	0.4676
Customer_02	1375	770	0.5600
Customer_03	700	280	0.4000
Customer_04	260	110	0.4231