Question

我创建了三个表名：emps，emp_project，emp_location

现在我想从每个表中选择一列，但到目前为止我执行连接查询时会出现此错误：

join子句中某个表达式的类型不正确。调用＆＃39; GroupJoin＆＃39;。
时，类型推断失败

我执行的查询是：

from e in Emp_info
from p in Emp_projects
join l in Emp_locations
    on new { e.User_id , p.Project_id  } equals new { l.User_id, l.Project_id  } into detail
from l in detail
select new
{
    e.Middlename,
    p.Project_name,
    l.Location
}; 
query.Dump("Join query");

不知道哪个条款导致错误！

Answer 1

我的猜测是，它试图比较的两个匿名类型是不一样的（也确保属性是相同的数据类型）。

更改

on new { e.User_id , p.Project_id  } equals new { l.User_id, l.Project_id  } into detail

要

 on new { UserId = e.User_id, ProjectId = p.Project_id  } equals 
    new { UserId = l.User_id, ProjectId = l.Project_id  } into detail

Answer 2

这对我有用

void Main()
{
    var Emp_info = new List<Info>
        {
            new Info {Middlename = "Middlename",User_id = "1"}
        };
    var Emp_projects = new List<Project>
        {
            new Project{Project_id = "1",Project_name = "Project"}
        };
    var Emp_locations = new List<LocationInfo>
        {
            new LocationInfo{Location = "Location",Project_id="1",User_id = "1"}
        };

    /* your code */
    var query = from e in Emp_info
    from p in Emp_projects
    join l in Emp_locations
        on new { e.User_id , p.Project_id  } equals new { l.User_id, l.Project_id  } into detail
    from l in detail
    select new
    {
        e.Middlename,
        p.Project_name,
        l.Location
    }; 

    query.Dump("Join query");
    /* your code */
}

class Info
{
    public string User_id;
    public string Middlename;
}

class Project
{
    public string Project_id;
    public string Project_name;
}

class LocationInfo
{
    public string User_id;
    public string Project_id;
    public string Location;
}

Answer 3

到目前为止，我已经通过一次使用多个连接来实现此解决方案。不是正确的做法。

var query=
        from e in Emp_info
        join l in Emp_locations on e.User_id  equals  l.User_id // first join
        join p in Emp_projects on l.Location_id equals p.Project_id    // second join
        select new
        {
            Name= e.Middlename,
            Project =  p.Project_name,
            Location = l.Location
        };

        query.Dump();

Answer 4

终于得到了答案..这完全符合我的要求

      var query=
from e in db.emp_info
from p in db.emp_projects
join l in db.emp_locations
    on new { username= e.User_id ,project_id=p.project_id } equals new { username=l.User_id,project_id= l.Project_id  } into detail
from l in detail
select new
{
   e,p,l
};
            foreach (var q in query)
            {
                Console.WriteLine("{0} has done project like {1} in {2}",q.e.Middlename,q.p.project_name,q.l.location);
            }
            Console.ReadLine();
                }
                Console.ReadLine();

linq中的加入条款

4 个答案: