MariaDB 10.3

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/* Sales */
CREATE TABLE sales (
    id int auto_increment primary key,
    category VARCHAR(255),
    sales_date DATE,
    sales_volume INT
);

INSERT INTO sales
(category, sales_date, sales_volume
)
VALUES 
("Category_01", "2020-01-03", "500"),
("Category_01", "2020-04-15", "700"),
("Category_01", "2020-08-27", "180"),
("Category_01", "2020-09-29", "320"),
("Category_02", "2020-07-17", "420"),
("Category_02", "2020-12-24", "350"),
("Category_02", "2020-12-08", "570"),
("Category_03", "2020-01-10", "180"),
("Category_03", "2020-10-20", "970");

/* Return Risk */
CREATE TABLE return_risk (
    id int auto_increment primary key,
    category VARCHAR(255),
    quarter VARCHAR(255),
    return_risk_rate DECIMAL(5,2)
);

INSERT INTO return_risk
(category, quarter, return_risk_rate
)
VALUES 
("Category_01", "2019-Q1", "0.40"),
("Category_01", "2019-Q2", "0.45"),
("Category_01", "2019-Q3", "0.52"),
("Category_01", "2019-Q4", "0.67"),
("Category_02", "2019-Q1", "0.32"),
("Category_02", "2019-Q2", "0.47"),
("Category_02", "2019-Q3", "0.25"),
("Category_02", "2019-Q4", "0.32"),
("Category_03", "2019-Q1", "0.40"),
("Category_03", "2019-Q2", "0.52"),
("Category_03", "2019-Q3", "0.48"),
("Category_03", "2019-Q4", "0.61");

You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near '%,
(s.category * r.return_risk_rate) AS return_risk_value
FROM sales s
JOIN r...' at line 4