Question

我正在尝试使用C＃代码中的条件创建左连接。我可以在SQL Server中编写它，但我在编写lambda表达式时遇到问题。这是我的SQL代码的一个粗略部分：

Select x.RequestId, aud.DepartmentId
From Requests x
Left Join UserDepartment ud on x.AssignedToTeam = ud.DepartmentId and ud.User = 'Joe'

我知道如何编写左连接，但我不熟悉如何或是否可以添加条件。

Answer 1

我怀疑你想要这样的东西：

var query = from request in db.Requests
            join department in db.UserDepartments
                                 .Where(dep => dep.User == "Joe")
              on request.AssignedToTeam equals department.DepartmentId
              into departments
            from dep in departments.DefaultIfEmpty()
            select new { request.RequestId,
                         DepartmentId = dep == null ? null : (int?) dep.DepartmentId
            };

（如果有必要，显然会改变你想要处理缺少部门ID的方式。）

Answer 2

查看MSDN文章：How to: Perform Left Outer Joins (C# Programming Guide)

Answer 3

var request = db.View.Join(
    db.UserDepartment.Where(w=>w.User=="Joe"),
    a=>a.AssignedToTeam, 
    b=>b.DepartmentId, 
    (a,b)=> new {View = a, UserDepartment = b});

左边使用Linq加入条件

3 个答案: