conditional order by to based on two columns with some limits

Question

I am stuck on a query..

I have a table of ads..There I have column sort_order and date_added.sort_order is set base on group type.So on my page say I want to show 12 ads by 4 groups.

I want 3 ads have order 1 at top in dec order by date_added.

Then 3 ads have order 2 in dec order by date_added.

Then 3 ads have order 3 in dec order by date_added.

Then 3 ads have order 4 in dec order by date_added.

so on front end it should look like this

Name    date_added sort_order
ad1      12-2-2015   1
ad2      11-2-2015   1
ad3      10-2-2015   1

ad4      13-2-2015   2
ad5      12-2-2015   2
ad6      07-2-2015   2

ad7      18-2-2015   3
ad8      10-2-2015   3
ad9      03-2-2015   3

ad10      12-2-2015   4
ad11      08-2-2015   4
ad12      03-2-2015   4

If sort_order=1 have only 2 ads then sort_order=2 can show 4 ads. Is it achievable by single query.Please give a advise.

What I am trying is

Select name,sort_order,date_added form ads order by sort_order ASC,date_added DESC LIMIT 0,12

Its not diving results in equal parts for each group.

Answer 1

This is easiest with union all :

select t.*
from ((Select name, sort_order, date_added 
       from ads
       where sort_order = 1
       order by date_added desc
       LIMIT 4
      ) union all
      (Select name, sort_order, date_added 
       from ads
       where sort_order = 2
       order by date_added desc
       LIMIT 4
      ) union all
      (Select name, sort_order, date_added 
       from ads
       where sort_order = 3
       order by date_added desc
       LIMIT 4
      ) union all
      (Select name, sort_order, date_added 
       from ads
       where sort_order = 4
       order by date_added desc
       LIMIT 4
      )
     ) t
order by sort_order, date_added desc;

However, this gets harder if you want 12 rows but might not have the intermediate rows. Allocating the extra values is challenging. Here is one approach:

select s.*
from (select s.*,
             (@rn := if(@so = sort_order, @rn + 1,
                        if(@so := sort_order, 1, 1)
                       )
             ) seqnum
      from ads s cross join
           (select @rn := 0, @so := 0) vars
      order by sort_order, date_added desc
     ) s
where @rn <= 12
order by (case when @rn <= 4 then @rn else @rn + sort_order + 100 end)
limit 12;