mysql query, how can i get top 5 downloads with GROUP_CONCAT

Question

the following data in MySql table.

brand_id|image_id|downloads
--------|--------|---------
8       |9       |8    
35      |2829    |4    
28      |2960    |3    
28      |2961    |3    
28      |3041    |3    
35      |2831    |3    
28      |2965    |3    
35      |2832    |3    
28      |2959    |2    
28      |2976    |2    
35      |2894    |2  

How can i find top 5 images with GROUP_CONCAT function in each brand And i would like to following to return

brand_id|image_ids                
--------|--------------------------
8       |9                       
35      |2829,2831,2832,2894     
28      |2960,2961,3041,2965,2959

The following query provide expected result

SELECT brand_id, SUBSTRING_INDEX(GROUP_CONCAT(image_id SEPARATOR ','),',',5) AS image_ids
FROM Table1
GROUP BY brand_id

i have a large amount data approx 2MILION records in table and brand_id, image_id both are repeated with each user download so when i run query i got following error

MySql Error: %d line(s) were cut by GROUP_CONCAT()

I think the GROUP_CONCAT range exceeded, can you please provide solution so i have get only top 5 downloads in each brand so there not require SUBSTRING_INDEX function.

Thank you

Answer 1

Try this:

SELECT brand_id, GROUP_CONCAT(image_id) image_ids
FROM (SELECT brand_id, image_id, 
             IF(@lastBrandId = @lastBrandId:=brand_id, @Idx:=@Idx+1, @Idx:=0) rowNumber 
      FROM Table1, (SELECT @lastBrandId:=0, @Idx:=0) A
      ORDER BY brand_id, downloads DESC
     ) AS A
WHERE rowNumber < 5
GROUP BY brand_id