在mysql 8.0.2及更高版本中，我们可以使用窗口函数。我们可以利用 Row_Number() 窗口函数，用于确定年和月的串联表达式的分区内的行号。分区内的排序是根据 activity .
然后我们可以使用这个结果集作为派生表，并考虑行数最多为5。这将给我们5行每月，有顶部 activity 价值观。

SELECT dt.* 
FROM 
(
  SELECT
    concat(m.firstname, " ",m.lastname) AS name,
    SEC_TO_TIME(SUM(TIME_TO_SEC(TIMEDIFF(pl.end_activity,pl.start_activity)))) AS activity,
    month(start_activity) AS month,
    year(start_activity) AS year, 
    ROW_NUMBER() OVER (PARTITION BY CONCAT(year(start_activity), month(start_activity))
                       ORDER BY SEC_TO_TIME(SUM(TIME_TO_SEC(TIMEDIFF(pl.end_activity,pl.start_activity)))) DESC) AS row_no 
  FROM 
    log AS pl
  INNER JOIN
    employee AS m
  ON
    m.employee = pl.employee
  GROUP BY
    name,
    year,
    month
) AS dt 
WHERE dt.row_no <= 5
ORDER BY
    dt.year,
    dt.month,
    dt.activity