mysql 动态列转行

CREATE TABLE sales (
    id INT PRIMARY KEY,
    product VARCHAR(50),
    january INT,
    february INT,
    march INT
);

SELECT id, product, 'january' AS month, january AS amount FROM sales
UNION ALL
SELECT id, product, 'february' AS month, february AS amount FROM sales
UNION ALL
SELECT id, product, 'march' AS month, march AS amount FROM sales;

SELECT id, product, JSON_UNQUOTE(JSON_EXTRACT(JSON_OBJECT('january', january, 'february', february, 'march', march), CONCAT('$."', month, '"'))) AS amount
FROM (
    SELECT id, product, 'january' AS month, january AS amount FROM sales
    UNION ALL
    SELECT id, product, 'february' AS month, february AS amount FROM sales
    UNION ALL
    SELECT id, product, 'march' AS month, march AS amount FROM sales
) AS subquery;