group_concat di SQL Server 2012 dengan ORDER BY kolom lain

Saya tidak yakin apakah ini akan lebih cepat, tetapi ini adalah versi alternatif di mana Anda tidak bergabung di purchases dua kali dalam STUFF() :

select customer_id,
  min(purchased_at) as first_purchased_at,
  stuff ((select ',' +  p2.product 
          from
          (
            select product, customer_id,
                ROW_NUMBER() over(partition by customer_id, product order by purchased_at) rn,
                ROW_NUMBER() over(partition by customer_id order by purchased_at) rnk   
            from purchases
          ) p2 
          where p2.customer_id = p1.customer_id
            and p2.rn = 1
          group by p2.product, rn, rnk
          order by rnk
          for XML PATH('') ), 1,1,'') AS all_purchased_products  
from purchases p1
group by customer_id;

Lihat SQL Fiddle dengan Demo

Hasil:

| CUSTOMER_ID |               FIRST_PURCHASED_AT | ALL_PURCHASED_PRODUCTS |
---------------------------------------------------------------------------
|           1 |      June, 01 2012 00:00:00+0000 |           apples,pears |
|           2 |      June, 01 2012 00:00:00+0000 |                 apples |
|           3 | September, 02 2012 00:00:00+0000 |   pears,apples,bananas |