How to find median value with group by (MySQL)

Question

Need to find median value of time difference between sent date and click date (in seconds) for each type of emails. I found solution just for all data:

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Answer 1

First, you need to enumerate the rows for each type. Using variables, this code looks like:

select sc.*,
       (@rn := if(@t = id_type, @rn + 1,
                  if(@t := id_type, 1, 1)
                 )
       ) as seqnum
from (select timestampdiff(second, s.date_sent, c.date_click) as time_diff,
             s.id_type,
      from emails_sent s inner join
           emails_clicks c
           on s.id = c.id_email
      order by time_diff
     ) sc cross join
     (select @t := -1, @rn := 0) as params;

Then, you need to bring in the total number for each type and do the calculation for the median:

select sc.id_type, avg(time_diff)
from (select sc.*,
             (@rn := if(@t = id_type, @rn + 1,
                        if(@t := id_type, 1, 1)
                       )
             ) as seqnum
      from (select timestampdiff(second, s.date_sent, c.date_click) as time_diff,
                   s.id_type,
            from emails_sent s inner join
                 emails_clicks c
                 on s.id = c.id_email
            order by time_diff
           ) sc cross join
           (select @t := -1, @rn := 0) as params
     ) sc join
     (select id_type, count(*) as cnt
      from emails_sent s inner join
           emails_clicks c
           on s.id = c.id_email
      group by id_type
     ) n
where 2 * seqnum in (n.cnt, n.cnt, n.cnt + 1, n.cnt + 2)
group by sc.id_type;