我有一个包含大约一百万个条目的表：
customer_id | purchased_at | product
1 | 2012-06-01 00:00 | apples
1 | 2012-09-02 00:00 | apples
1 | 2012-10-01 00:00 | pears
2 | 2012-06-01 00:00 | apples
2 | 2012-07-01 00:00 | apples
3 | 2012-09-02 00:00 | pears
3 | 2012-10-01 00:00 | apples
3 | 2012-10-01 01:00 | bananas
我想将产品连接到一行，DISTINCT以及purchase_at的顺序
在MySQL中我只使用
select customer_id, min(purchased_at) as first_purchased_at,
group_concat(DISTINCT product order by purchased_at) as all_purchased_products
from purchases group by customer_id;
获取
customer_id | first_purchased_at | all_purchased_products
1 | 2012-06-01 00:00 | apples, pears
2 | 2012-06-01 00:00 | apples
3 | 2012-09-02 00:00 | pears, apples, bananas
如何在SQL Server 2012中执行此操作？
我尝试了下面的'hack'，这有效，但是它太过分了，在长桌上表现不佳
select
customer_id,
min(purchased_at) as first_purchased_at,
stuff ( ( select ',' + p3.product
from (select p2.product, p2.purchased_at,
row_number() over(partition by p2.product order by p2.purchased_at) as seq
from purchases p2 where
p2.customer_id = p1.customer_id ) p3
where p3.seq = 1 order by p3.purchased_at
for XML PATH('') ), 1,1,'') AS all_purchased_products
from purchases p1
group by customer_id;
我该怎么做才能解决这个问题？
答案 0 :(得分：5)
我不确定这是否会更快，但这是一个替代版本，您不会在purchases STUFF()两次加入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;
：
| 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 |
结果：
{{1}}