Getting the latest n records for each group

问题

Lets say I have the following table:

id  coulmn_id  value    date
1      10      'a'     2016-04-01
1      11      'b'     2015-10-02
1      12      'a'     2016-07-03
1      13      'a'     2015-11-11
2      11      'c'     2016-01-10
2      23      'd'     2016-01-11
3      11      'c'     2016-01-09
3      111     'd'     2016-01-11
3      222      'c'     2016-01-10
3      333      'd'     2016-01-11

for n = 3, I want to get the latest n records<=3 for each id. So I will have the following output:

id  column_id  value    date
1      10        'a'     2016-04-01
1      12        'a'     2016-07-03
1      13        'a'     2015-11-11
2      11        'c'     2016-01-10
2      23        'd'     2016-01-11
3      111       'd'     2016-01-11
3      222       'c'     2016-01-10
3      333       'd'     2016-01-11

回答1:

I am answering because the referenced question has an unstable answer (I'll comment on that there).

Here is a solution that should work:

select t.*
from (select t.*,
             (@rn := if(@id = id, @rn + 1,
                        if(@id := id, 1, 1)
                       )
             ) as seqnum
      from t cross join
           (select @rn := 0, @id := -1) params
      order by id, date desc
     ) t
where seqnum <= 3;

The difference in the solutions is that the variable assignments are all in a single expression. MySQL does not guarantee the order of evaluation of expressions, so this is very important if the code is going to work consistently.

回答2:

You could do this with the use of variables. First go through the results in reverse order and assign a row number, then filter the results for row numbers less or equal to 3, and re-order:

select   id, value, date
from     (
           select      id, value, date,
                       @rn := if(@id = id, @rn+1, if (@id := id, 1, 1)) rn
           from        mytable,
           cross join  (@id := null, @rn := null) init
           order by    id, date desc
         ) as base
where    rn <= 3
order by id, date asc

来源：https://stackoverflow.com/questions/38535020/getting-the-latest-n-records-for-each-group