Linq to sql之left join运用示例

示例一：

var l= from a in cardsBll.GetCards()
	  join b in usersBll.GetAllUsers()
	  on a.CardSn equals b.CardSn into temp
	  where a.CardSn.Contains(key)
	  from t in temp.DefaultIfEmpty()
	  select new CardsAndUsersView
	  {
		  Card_Id = a.Id,
		  CardPw = a.CardPw,
		  Expries = a.Expries,
		  IsEnabled = a.IsEnabled,
		  CardSn = a.CardSn,
		  CreateDate = a.CreateDate,
		  ModifyDate = a.ModifyDate,
		  User_Id = t==null?0:t.Id,
		  Province = t == null ? "" : t.Province,
		  City = t == null ? "" :t.City,
		  AreaOrCounty = t == null ? "" : t.AreaOrCounty,
		  AddressInfo = t == null ? "" : t.AddressInfo
	  };

生成的SQL如下：

exec sp_executesql N‘SELECT TOP (20)
[Project1].[Id] AS [Id],
[Project1].[CardPw] AS [CardPw],
[Project1].[Expries] AS [Expries],
[Project1].[IsEnabled] AS [IsEnabled],
[Project1].[CardSn] AS [CardSn],
[Project1].[CreateDate] AS [CreateDate],
[Project1].[ModifyDate] AS [ModifyDate],
[Project1].[C1] AS [C1],
[Project1].[C2] AS [C2],
[Project1].[C3] AS [C3],
[Project1].[C4] AS [C4],
[Project1].[C5] AS [C5]
FROM ( SELECT [Project1].[Id] AS [Id], [Project1].[CardSn] AS [CardSn], [Project1].[CardPw] AS [CardPw], [Project1].[IsEnabled] AS [IsEnabled], [Project1].[Expries] AS [Expries], [Project1].[CreateDate] AS [CreateDate], [Project1].[ModifyDate] AS [ModifyDate], [Project1].[C1] AS [C1], [Project1].[C2] AS [C2], [Project1].[C3] AS [C3], [Project1].[C4] AS [C4], [Project1].[C5] AS [C5], row_number() OVER (ORDER BY [Project1].[Id] ASC) AS [row_number]
	FROM ( SELECT
		[Extent1].[Id] AS [Id],
		[Extent1].[CardSn] AS [CardSn],
		[Extent1].[CardPw] AS [CardPw],
		[Extent1].[IsEnabled] AS [IsEnabled],
		[Extent1].[Expries] AS [Expries],
		[Extent1].[CreateDate] AS [CreateDate],
		[Extent1].[ModifyDate] AS [ModifyDate],
		CASE WHEN ([Extent2].[Id] IS NULL) THEN 0 ELSE [Extent2].[Id] END AS [C1],
		CASE WHEN ([Extent2].[Id] IS NULL) THEN N‘‘‘‘ ELSE [Extent2].[Province] END AS [C2],
		CASE WHEN ([Extent2].[Id] IS NULL) THEN N‘‘‘‘ ELSE [Extent2].[City] END AS [C3],
		CASE WHEN ([Extent2].[Id] IS NULL) THEN N‘‘‘‘ ELSE [Extent2].[AreaOrCounty] END AS [C4],
		CASE WHEN ([Extent2].[Id] IS NULL) THEN N‘‘‘‘ ELSE [Extent2].[AddressInfo] END AS [C5]
		FROM  [dbo].[Cards] AS [Extent1]
		LEFT OUTER JOIN [dbo].[UserInfo] AS [Extent2] ON ([Extent1].[CardSn] = [Extent2].[CardSn]) OR (([Extent1].[CardSn] IS NULL) AND ([Extent2].[CardSn] IS NULL))
		WHERE (1 = [Extent1].[IsEnabled]) AND ([Extent1].[CardSn] LIKE @p__linq__0 ESCAPE N‘‘~‘‘)
	)  AS [Project1]
)  AS [Project1]
WHERE [Project1].[row_number] > 0
ORDER BY [Project1].[Id] ASC‘,N‘@p__linq__0 nvarchar(4000)‘,@p__linq__0=N‘%%‘

执行结果：

页面效果：

示例二：

DataClasses1DataContext db = new DataClasses1DataContext();
var leftJoinSql = from student in db.Student
join book in db.Book on student.ID equals book.StudentID into temp
from tt in temp.DefaultIfEmpty()
select new
{
	sname= student.Name,
	bname = tt==null?"":tt.Name
};

原文地址：https://www.cnblogs.com/zhaow/p/9796526.html