跨三个表的 Linq Join答案

【问题标题】：Linq Join across three tables跨三个表的 Linq Join
【发布时间】：2010-07-19 20:08:41
【问题描述】：

我有三个表，需要编写一个 linq 查询，该查询基于两个不同的连接从所有活动中提取。

我需要 ActivityVolunteers 中存在志愿者的所有活动
我需要OrganizationID 在Activity 表中并且志愿者属于组织(OrganizationVolunteers) 但不在ActivityVolunteers 中的所有活动。志愿者可能不属于活动，但属于组织。

下面是我尝试编写 Linq 的表结构。

活动

活动 ID
活动名称
组织 ID

活动志愿者

活动 ID
志愿者编号
活动角色

组织志愿者

组织 ID
志愿者编号

这是我在 linq 上的弱尝试，我无法弄清楚加入两个结果集以仅获得独特的活动。

from a in Activities
join av in ActivityVolunteers on a.ActivityID equals av.ActivityID 
where av.VolunteerID==1
select new 
    {
        a.ActivityID,
        a.ActivityName,
        av.ActivityRole,
        a.OrganizationID
    }

from org in (from a in Activities
join ov in OrganizationVolunteers on a.OrganizationID equals ov.OrganizationID
where ov.VolunteerID==1
select new 
    {
        a.ActivityID,
        a.ActivityName,
        ActivityRole = "Prospect",
        a.OrganizationID
    })
select org

我尝试了一个联合，但它正在复制记录。

(from a in Activities
join av in ActivityVolunteers on a.ActivityID equals av.ActivityID 
where av.VolunteerID==1
select new 
    {
        a.ActivityID,
        a.ActivityName,
        av.ActivityRole,
        a.OrganizationID,
        OrganizationName = (from o in Organizations where o.OrganizationID == a.OrganizationID select o.OrganizationName).FirstOrDefault()
    }).Union
    (from a in Activities
join ov in OrganizationVolunteers on a.OrganizationID equals ov.OrganizationID
where ov.VolunteerID==1
select new 
    {
        a.ActivityID,
        a.ActivityName,
        ActivityRole = "Prospect",
        a.OrganizationID,
        OrganizationName = (from o in Organizations where o.OrganizationID == a.OrganizationID select o.OrganizationName).FirstOrDefault()
    })

【问题讨论】：

我想我正在寻找一个只提供不同的联合。
Union 提供了不同的答案...
这可能有用：stackoverflow.com/questions/8900077/…

标签： linq

【解决方案1】：

(from a in Activities
join ov in OrganizationVolunteers on a.OrganizationID equals ov.OrganizationID
join av in ActivityVolunteers
on a.ActivityID equals av.VolunteerID into JoinedActVol
from av in JoinedActVol.DefaultIfEmpty()
where ov.VolunteerID==1
select new 
    {
        a.ActivityID,
        a.ActivityName,
        av.ActivityRole,
        a.OrganizationID,
        OrganizationName = (from o in Organizations where o.OrganizationID == a.OrganizationID select o.OrganizationName).FirstOrDefault()
    }).Union
    (from a in Activities
join av in ActivityVolunteers on a.ActivityID equals av.ActivityID 
where av.VolunteerID==1 && a.OrganizationID == null
select new 
    {
        a.ActivityID,
        a.ActivityName,
        av.ActivityRole,
        a.OrganizationID,
        OrganizationName = ""
    })

【讨论】：