LINQ的左連接、右連接、內連接和Lamda表達式實現Left join

1、左連接：

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、右連接：

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、內連接：

 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，它能夠為實序列提供一個默認的元素。DefaultIfEmpty使用了泛型中的default關鍵字。default關鍵字對於引用類型將返回null,而對於值類型則返回0。對於結構體類型，則會根據其成員類型將它們相應地初始化為null(引用類型)或0(值類型)

 

我們可以不使用default關鍵字，但在要DefaultIfEmpty中給定當空時的默認對象值。

 

Lamda表達式實現Left join

 

public class AnalysisPlayer
{
        /// <summary>
        /// 玩家名稱
        /// </summary>
        public string PlayerName { get; set; }
        /// <summary>
        /// 玩家賬號
        /// </summary>
        public string Account { get; set; }
        /// <summary>
        /// 注冊時間
        /// </summary>
        public DateTime RegTime { get; set; }
        /// <summary>
        /// 充值總數
        /// </summary>
        public int Points { get; set; }
}
------解決方案--------------------
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 });