Linq Join

学生集合：

private static List<Student> stu_li = new List<Student>
{
    new Student { UserId="1",Name="Tom",Age=25,DeptId="11"},
    new Student { UserId="2",Name="Cos",Age=26,DeptId="11"},
    new Student { UserId="3",Name="Tan",Age=27,DeptId="12"},
    new Student { UserId="4",Name="Sin",Age=28,DeptId="13"}
};    

学部集合：

private static List<Deptment> dept_li = new List<Deptment>
{
    new Deptment { DeptId="11",DeptName="信息学部",Description="" } 
};

=========================================================================
left join
=========================================================================
说明：如果学生中的DeptId在学部中不存在的话，请在代码①行中加上new Deptment()，否则会报错。

public static void LinqLeftJoin()
{
    var data_left_join = from stu in stu_li
                                 join dept in dept_li on stu.DeptId equals dept.DeptId into depts
                                 from dept_tmp in depts.DefaultIfEmpty(new Deptment())//①
                                 select new { StudentName = stu.Name, Age = stu.Age, DeptName = dept_tmp.DeptName };

    string str = JsonConvert.SerializeObject(data_left_join);
    Console.WriteLine(str);
}

结果：

[
    {
        "StudentName":"Tom",
        "Age":25,
        "DeptName":"信息学部"
    },
    {
        "StudentName":"Cos",
        "Age":26,
        "DeptName":"信息学部"
    },
    {
        "StudentName":"Tan",
        "Age":27,
        "DeptName":null
    },
    {
        "StudentName":"Sin",
        "Age":28,
        "DeptName":null
    }
]

=========================================================================
inner join
=========================================================================

 public static void LinqInnerJoin()
 {
     var data_left_join = from stu in stu_li
                          join dept in dept_li on stu.DeptId equals dept.DeptId
                          select new { StudentName = stu.Name, Age = stu.Age, DeptName = dept.DeptName };

      string str = JsonConvert.SerializeObject(data_left_join);
      Console.WriteLine(str);
 }

结果：

[
    {
        "StudentName":"Tom",
        "Age":25,
        "DeptName":"信息学部"
    },
    {
        "StudentName":"Cos",
        "Age":26,
        "DeptName":"信息学部"
    }
]

==========================================================================
cross join
==========================================================================
//在学部集合中新加了一行数据：

private static List<Deptment> dept_li = new List<Deptment>
{
     new Deptment { DeptId="11",DeptName="信息学部",Description="" },
     new Deptment { DeptId="16",DeptName="社科学部",Description="" },
5 };

public static void LinqCrossJoin()
{
            var data_left_join = from stu in stu_li
                                 from dept in dept_li
                                 select new
                                 {
                                     StudentName = stu.Name,
                                     Age = stu.Age,
                                     DeptName = dept.DeptName,
                                     Description = dept.Description
                                 };

            string str = JsonConvert.SerializeObject(data_left_join);
            Console.WriteLine(str);
}

结果：

[
    {
        "StudentName":"Tom",
        "Age":25,
        "DeptName":"信息学部",
        "Description":""
    },
    {
        "StudentName":"Tom",
        "Age":25,
        "DeptName":"社科学部",
        "Description":""
    },
    {
        "StudentName":"Cos",
        "Age":26,
        "DeptName":"信息学部",
        "Description":""
    },
    {
        "StudentName":"Cos",
        "Age":26,
        "DeptName":"社科学部",
        "Description":""
    },
    {
        "StudentName":"Tan",
        "Age":27,
        "DeptName":"信息学部",
        "Description":""
    },
    {
        "StudentName":"Tan",
        "Age":27,
        "DeptName":"社科学部",
        "Description":""
    },
    {
        "StudentName":"Sin",
        "Age":28,
        "DeptName":"信息学部",
        "Description":""
    },
    {
        "StudentName":"Sin",
        "Age":28,
        "DeptName":"社科学部",
        "Description":""
    }
]

=========================================================================

 /// <summary>
    /// 学生
    /// </summary>
    public class Student
    {
        public string UserId { get; set; }
        public string Name { get; set; }
        public int Age { get; set; }

        public string DeptId { set; get; }
    }

    /// <summary>
    /// 学部
    /// </summary>
    public class Deptment
    {
        public string DeptId { get; set; }
        public string DeptName { get; set; }
        public string Description { get; set; }
    }