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set @sum=0;

select mea1_count, num_members,  @sum:=@sum+num_members from (
  SELECT
    mea1_count,
    count(mem_id) num_members
  FROM (
         SELECT
           mea1.mem_id,
           count(mea1.id) mea1_count
         FROM member_email_actions mea1
         WHERE mea1.open_date BETWEEN '2016-05-02' AND '2016-05-09'
               AND mea1.mem_id IN (
           SELECT m.id
           FROM member m
           WHERE date(m.activated_at) = '2016-05-01'
         )
         GROUP BY mea1.mem_id
       ) AS x
  GROUP BY mea1_count
  ORDER BY mea1_count DESC
) y

;