mysql - select recent activity per user into column

Question

Edit - I found it different from pivot table. Because in my case, it based on date

I have table with linked data. I want to return the most recent activity per customer into column, like temporal data for data mining purpose.

So I have these columns in my table:

id_user | item_id | date

      1 |       2 | 2016-11-10
      1 |       3 | 2016-9-9
      1 |      23 | 2016-8-8
      1 |      21 | 2016-6-6
      1 |       5 | 2016-7-7
      1 |       4 | 2016-10-10
      2 |       3 | 2016-9-9
      2 |       4 | 2016-10-10
      2 |      21 | 2016-5-4
      3 |       4 | 2016-10-10
      3 |       4 | 2016-9-9

First - I want to filter data to get "recent 5" activity. Then I comes up with this code

set @num := 0, @group := '';

select x.`id_user`, x.`item_id`, x.`date`
from
(
   select `id_user`, `item_id`, `date`,
      @num := if(@group = `id_user`, @num + 1, 1) as row_number,
      @group := `id_user` as dummy
  from ratings
  order by `id_user`, `date` desc, `item_id`
) as x,
where x.row_number <= 5
ORDER BY x.`user_id`, x.date DESC;

Those code give me this table:

 id_user | item_id | date

       1 |       2 | 2016-11-10
       1 |       4 | 2016-10-10
       1 |       3 | 2016-9-9
       1 |      23 | 2016-8-8
       1 |       5 | 2016-7-7
       2 |       4 | 2016-10-10
       2 |       3 | 2016-9-9
       2 |      21 | 2016-5-4
       3 |       4 | 2016-10-10
       3 |       4 | 2016-9-9

BUT - I want something like this for data-mining puspose

Id_user | item_1 | Item_2 | Item_3 | Item_4 | Item_5

      1 |      2 |      4 |      3 |     23 |      5 |
      2 |      4 |      3 |     21 |   NULL |   NULL |
      3 |      4 |      4 |   NULL |   NULL |   NULL |

Did you get my idea? Sorry, if I can't explain it clearly, hope you can understand what I want.

The Question

How can I make the SQL query for these issue? (currently I got RAW Data from MySQL database)
Is there any better approach for this issue? I hope it's a best practice.

Consider using `GROUP_CONCAT()` instead of "pivot". – Rick James Oct 16 '16 at 05:21 — Rick James, Oct 16 '16 at 05:21

score 0 · Answer 1 · answered Oct 11 '16 at 11:09

First, when using variables, do not assign and use the variables in different expressions. MySQL does not guarantee the order of evaluation of expressions in a SELECT. And, in fact, under some circumstances, evaluates them in a different order.

So, this should be your query:

select r.`id_user`, r.`item_id`, r.`date`
from (select `id_user`, `item_id`, `date`,
             (@num := if(@u = id_user, @num + 1,
                         if(@u := id_user, 1, 1)
                        )
              ) as num
      from ratings r cross join
           (select @num := 0, @u := -1) params
      order by `id_user`, `date` desc, `item_id`
     ) r
where x.row_number <= 5
order by x.`user_id`, x.date desc;

Then, just use conditional aggregation:

select r.`id_user`,
       max(case when num = 1 then r.item_id end) as item_id_1,
       max(case when num = 2 then r.item_id end) as item_id_2,
       max(case when num = 3 then r.item_id end) as item_id_3,
       max(case when num = 4 then r.item_id end) as item_id_4,
       max(case when num = 5 then r.item_id end) as item_id_5
from (select `id_user`, `item_id`, `date`,
             (@num := if(@u = id_user, @num + 1,
                         if(@u := id_user, 1, 1)
                        )
              ) as num
      from ratings r cross join
           (select @num := 0, @u := -1) params
      order by `id_user`, `date` desc, `item_id`
     ) r
where x.row_number <= 5
group by x.`user_id`;

mysql - select recent activity per user into column

1 Answers1