MySQL的TopN SQL的写法

比如取group by后每个分组的Top 3

SELECT
    mac_group,
    mac,
    dur
FROM
(
    SELECT
        mac_group,
        sum(sum_dur) as dur,
        mac,
        @rn := IF(@prev = mac_group and @prev_mac = mac, @rn + 1, 1) AS rn,
        @prev := mac_group,
        @prev_mac := mac
    FROM mac_customer_stats
    JOIN (SELECT @prev := NULL, @prev_mac :=NULL, @rn := 0) AS vars
    where mac_group in (1,2,3)
    group by mac_group, mac, day
    ORDER BY mac_group, mac, dur DESC
) AS T1
WHERE rn <= 3;