选择内部联接上特定列的不同

Question

我有一个表格，表示 Offer、Profiles 和 Skills 之间的三元关系。

任何给定的优惠都可以有多个个人资料，以及多个相关的技能。

类似这样的：

三元关系表

id_Offer    -   id_Profile  -   id_Skill
1           -   1           -   1
1           -   1           -   2
1           -   1           -   3
1           -   2           -   1
2           -   1           -   1
2           -   1           -   2
2           -   1           -   3
2           -   2           -   1

报价表

Offer   -   business_name
1       -   business-1
1       -   business-1
1       -   business-1
1       -   business-1
2       -   business-2 
2       -   business-2 
2       -   business-2 
2       -   business-2 

我想按个人资料进行查询过滤，并且只计算一次优惠，无论它关联了多少技能。

我正在执行类似以下查询的操作：

SELECT business_name, COUNT(*)
FROM Offer INNER JOIN
     ternary_relationship
     ON Offer.id_Offer = ternary_relationship.id_Offer AND
        id_Profile =  '1'
 GROUP BY business_name
 ORDER BY COUNT(*) DESC;

我已经看到 several possible 解决方案，但我无法让任何人为我的案子工作。当我为 id_Offer 命名或仅按 id_Offer 过滤时，它都不起作用。我总是在某处得到重复的条目。

Answer 1

您可以使用JOINS 并一次性使用DISTINCT

select o.business_name, count(distinct tr.id_Offer) counts
from Offer o inner join 
     ternary_relationship tr on 
     o.Offer = tr.id_Offer 
where tr.id_Profile = ?
group by o.business_name
order by 2 desc;