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我对如何获得购买数量和每件商品的投票数感到困惑。

执行两个简单的 JOIN 来连接以下 3 个表将乘以表purchase_count中的条目数Votes：

SELECT item_id, item_name, sum(backing_status) purchase_count, sum(vote) vote_count 
FROM Items
LEFT JOIN Purchases ON Items.item_id=Purchases.item_id
LEFT JOIN Votes ON Items.item_id=Votes.item_id
where purchase_status='bought' 
group by Items.item_id 

Table Items
item_id item_name
1           item_1
2           item_2
3           item_3

Table Purchases
item_id purchase_status 
1           bought          
2           bought          
1           bought          

Table Votes
item_id    vote 
1           1         
2           1          
3           -1         
1           -1
1           -1

所需的输出是：

item_id item_name  purchase_count vote_count
1           item_1      2          -1 //sum of -2 downvotes + 1 upvote
2           item_2      1           1
3           item_3      0          -1

我可能需要子查询，但不能完全弄清楚查询。或者，还有更好的方法？

4

2 回答 2

1

您应该能够使用以下代码，该代码使用带有两个聚合的子查询来获得结果：

select i.item_id,
  i.item_name,
  coalesce(p.TotalBought, 0)TotalBought,
  coalesce(v.vote_count, 0) vote_count
from items i
left join
(
  select item_id, count(*) TotalBought
  from purchases
  where purchase_status = 'bought'
  group by item_id
) p
  on i.item_id = p.item_id
left join
(
  select item_id, sum(vote) vote_count
  from votes
  group by item_id
) v
  on i.item_id = v.item_id;

请参阅SQL Fiddle with Demo。

您也可以在没有两个子查询的情况下编写此代码，但您需要将 WHERE 过滤器purchase_status放在 JOIN 上：

select i.item_id,
  i.item_name,
  count(p.item_id) TotalBought,
  coalesce(v.vote_count, 0) vote_count
from items i
left join purchases p
  on i.item_id = p.item_id
left join
(
  select item_id, sum(vote) vote_count
  from votes
  group by item_id
) v
  on i.item_id = v.item_id
group by i.item_id, i.item_name;

请参阅带有演示的 SQL Fiddle

于 2013-06-11T22:06:10.627 回答
0 
SELECT item_id, item_name, sum(backing_status) AS purchase_count, SUM(vote) 
    AS vote_count
FROM Items i
LEFT JOIN Purchases p on i.item_id = p.item_id
LEFT JOIN (SELECT SUM(vote), item_id FROM votes GROUP BY item_id) AS grv 
    on grv.item_id = i.item_id
WHERE purchase_status='bought';
于 2013-06-11T22:06:46.400 回答