对2楼格式美化了一下 (千万别 '拿来即用' 🤐)

SELECT
    r2.bid,
    r2.vala,
    r2.valb,
    r2.number,
    r2.aid,
    r2.mainNumber,
    r1.sum_ab AS a
FROM
    (
        SELECT
            b.id AS bid,
            b.vala,
            b.valb,
            b.number,
            a.id AS aid,
            a.mainNumber,
            b.vala * b.valb AS a
        FROM
            b
        LEFT JOIN a ON a.id = b.aid
    ) r2
INNER JOIN (
    SELECT
        *, SUM(ab) AS sum_ab
    FROM
        (
            SELECT
                b.id AS bid,
                b.vala,
                b.valb,
                b.number,
                a.id AS aid,
                a.mainNumber,
                b.vala * b.valb AS ab
            FROM
                b
            LEFT JOIN a ON a.id = b.aid
        ) AS c
    GROUP BY
        c.mainNumber
) AS r1 ON r1.mainNumber = r2.mainNumber;