[LINQ] join多个条件

[symbol_bc]

public class Student
{
 int ID {get; set;}
 int UID {get; set;}
 string Name {set; get;}
}

List<Student> s1 = new List<Student>();
List<Student> s2 = new List<Student>();

join规则 s1.ID == s2.UID && s1.UID == s2.ID
怎么写？

from g in s1
                           join f in s2 on new { s1.ID, s1.UID } equals new { s2.UID, s2.ID } into h
                           select g;
这样不对！

var interace = from g in s1
                           join f in s2 on new { s1.UserID, s1.PartnerUserID } equals new { s2.PartnerUserID, s2.UserID } into h
                           from c in h.DefaultIfEmpty()
                           select new
                           {
                               g
                           };

这样写也不对！

应该怎么写？？？

[机器人]

如果 into h 的话，要加上：

var query = from e1 in s1
join e2 in s2
on new { e1.ID, e1.UID } equals new { e2.ID, e2.UID } into h
from s in h
select s;

[symbol_bc]

[Quote=引用 10 楼 fangxinggood 的回复:]
@_@ 不会吧。

我这里没问题。你的class Student属性的Public加上了吗？
[/Quote]

哦，我知道了，new内部的程序顺序必须一致，才可以，可是这样一致对比的是相同
名字的成员，而我想比的是id和uid。不错！

[q107770540]

try:

	var query=from g in s1
		          join f in s2 
		          on new { ID1=g.ID, ID2=g.UID }  equals new { ID1=f.UID, ID2=f.ID } into h
				  from h1 in h.DefaultIfEmpty()
	           	 select h1;

[q107770540]

void Main()
{
        var s1 = GetData1();
	     var s2 = GetData2();

		var query=from g in s1
		          join f in s2 
		          on new { ID1=g.ID, ID2=g.UID }  equals new { ID1=f.UID, ID2=f.ID } into h
				  from h1 in h.DefaultIfEmpty()
	           	 select h1;
			 
     query.ToList().ForEach(q=>Console.WriteLine(q==null?"":q.ToString()));
		
 /*
 [ID:2],[UID:1],[Name:Data1_2]

[ID:2],[UID:3],[Name:Data1_3]

*/
	
}

 List<Student> GetData1()
		{
			var data = new List<Student>
						   {
							   new Student {ID = 1, UID = 2, Name = "Data1_1"},
							   new Student {ID = 2, UID = 2, Name = "Data1_2"},
							   new Student {ID = 3, UID = 2, Name = "Data1_3"},
							   new Student {ID = 4, UID = 4, Name = "Data1_4"}
						   };
			return data;
		}

		 List<Student> GetData2()
		{
			var data = new List<Student>
						   {
							   new Student {ID = 2, UID = 1, Name = "Data1_2"},
							   new Student {ID = 2, UID = 3, Name = "Data1_3"},
							   new Student {ID = 5, UID = 5, Name = "Data1_5"}
						   };
			return data;
		}
class Student
	{
		public int ID { get; set; }
		public int UID { get; set; }
		public string Name { set; get; }
		public override string ToString()
		{
			return string.Format("[ID:{0}],[UID:{1}],[Name:{2}]", ID, UID, Name);
		}
	}

[q107770540]

var query=from g in s1
		 join f in s2 
		 on new { ID1=g.ID, ID2=g.UID }  equals new { ID1=f.UID, ID2=f.ID } into h
		 select g;

[以专业开发人员为伍]

var interace = from g in s1
            join f in s2 on g.ID  equals  f.UID  into h
            from c in h.DefaultIfEmpty()
            where c!=null && g.UID== c.ID 
            select new
            {
                g
            };

不过，把你的原始class定义代码写好，你的那个根本没法编译通过。

[以专业开发人员为伍]

实际上，当有两条件需要比较，那么哪一个写在前边、哪一个写在后边，哪一个变量写在表达式左边、哪一个写在右边，这是要根据索引状况进行分析的。有索引的应该写在左边，并且首先写在前边的表达式得到的匹配结果记录应该尽可能少。

[以专业开发人员为伍]

嗯，上面你的左连接条件中的&&写错了，应该为

var interace = from g in s1
            join f in s2 on g.ID  equals  f.UID  into h
            from c in h.DefaultIfEmpty()
            where c==null || g.UID== c.ID 
            select new
            {
                g
            };

[symbol_bc]

[Quote=引用 1 楼 q107770540 的回复:]
C# code

var query=from g in s1
join f in s2
on new { ID1=g.ID, ID2=g.UID } equals new { ID1=f.UID, ID2=f.ID } into h
select g;
[/Quote]

弄了好长时间，原来是没有给他们重命名，就是说要为他们提供一个显式的映射关系，我还以为他们可以根据顺序自己映射呢？是这样吧，Tim。

[q107770540]

是的 可以这么理解

[机器人]

class Program
    {
        static void Main(string[] args)
        {
            var s1 = GetData1();
            var s2 = GetData2();

            var query = from e1 in s1
                        join e2 in s2
                            on new { e1.ID, e1.UID } equals new { e2.ID, e2.UID }
                        select e1;

            foreach (var e in query.ToList())
                Console.WriteLine(e.ToString());

            Console.Read();
        }

        static List<Student> GetData1()
        {
            var data = new List<Student>
                           {
                               new Student {ID = 1, UID = 1, Name = "Data1_1"},
                               new Student {ID = 2, UID = 2, Name = "Data1_2"},
                               new Student {ID = 3, UID = 3, Name = "Data1_3"},
                               new Student {ID = 4, UID = 4, Name = "Data1_4"}
                           };
            return data;
        }

        static List<Student> GetData2()
        {
            var data = new List<Student>
                           {
                               new Student {ID = 2, UID = 2, Name = "Data1_2"},
                               new Student {ID = 3, UID = 3, Name = "Data1_3"},
                               new Student {ID = 5, UID = 5, Name = "Data1_5"}
                           };
            return data;
        }
    }

    class Student
    {
        public int ID { get; set; }
        public int UID { get; set; }
        public string Name { set; get; }
        public override string ToString()
        {
            return string.Format("[ID:{0}],[UID:{1}],[Name:{2}]", ID, UID, Name);
        }
    }

[机器人]

这里不要 into h
select g 不会从h里取。

[symbol_bc]

[Quote=引用 7 楼 fangxinggood 的回复:]
C# code
class Program
{
static void Main(string[] args)
{
var s1 = GetData1();
var s2 = GetData2();

var query = from e1 in s1
……
[/Quote]

你这个能编译通过吗？
我在这里
var query = from e1 in s1
join e2 in s2
on new { e1.ID, e1.UID } equals new { e2.ID, e2.UID }
select e1;

就说join的调用推理类型失败！

[机器人]

@_@ 不会吧。

我这里没问题。你的class Student属性的Public加上了吗？

[symbol_bc]

对了，tim，这默认是内连接吧，怎样实现左连接