1、左连接：

var LeftJoin = from t1 in l1

join t2 in l2

on t1.ID equals t2.ID into Joinedt12

from t3 in Joinedt12.DefaultIfEmpty()

select new                        

{

Name = t1.Name,

Age=t2.Age                      

};

 

2、右连接：

var RightJoin = from t2 in l2

join t1in l1

on t2.ID equals t1.ID into joint12

from t3 in joint12.DefaultIfEmpty()

select new                          

{

Name = t1.Name,

Age=t2.Age  

};

 

3、内连接：

 var query = from t1 in l1

                 join t2 in l2

                 on t1.ID equals t2.ID

                 select new t3

                 {

                    USER_ID = t1.USER_ID,

                    USER_NAME = t1.USER_NAME,

                    USER_PASSWORD = t1.USER_PASSWORD,

                 };

 

 

注：上例中使用了DefaultIfEmpty操作符，它能够为实序列提供一个默认的元素。DefaultIfEmpty使用了泛型中的default关键字。default关键字对于引用类型将返回null,而对于值类型则返回0。对于结构体类型，则会根据其成员类型将它们相应地初始化为null(引用类型)或0(值类型)

 

我们可以不使用default，但在要DefaultIfEmpty中给定当空时的默认对象值。

 

Lamda表达式实现Left join

 

public class AnalysisPlayer{        /// 
        /// 玩家名称        /// 
        public string PlayerName { get; set; }        /// 
        /// 玩家账号        /// 
        public string Account { get; set; }        /// 
        /// 注册时间        /// 
        public DateTime RegTime { get; set; }        /// 
        /// 充值总数        /// 
        public int Points { get; set; }} ------解决方案-------------------- var query = User.GroupJoin(Charge, u => u.Account, c => c.Account, (u, c) => new { u, c }) .Select(data => new AnalysisPlayer { Account = data.u.Account, PlayerName = data.u.Name,   RegTime = data.u.CreateTime ?? DateTime.Now, Points =data.c==null?0:data.c.Points });