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关键字，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
{
        /// <summary>
        /// 玩家名称
        /// </summary>
        public string PlayerName { get; set; }
        /// <summary>
        /// 玩家账号
        /// </summary>
        public string Account { get; set; }
        /// <summary>
        /// 注册时间
        /// </summary>
        public DateTime RegTime { get; set; }
        /// <summary>
        /// 充值总数
        /// </summary>
        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 =http://www.mamicode.com/> new AnalysisPlayer { Account = data.u.Account, PlayerName = data.u.Name,  
RegTime = data.u.CreateTime ?? DateTime.Now, Points =data.c==null?0:data.c.Points });

LINQ的左连接、右连接、内连接和Lamda表达式实现Left join