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  • Linq表连接大全(INNER JOIN、LEFT OUTER JOIN、RIGHT OUTER JOIN、FULL OUTER JOIN、CROSS JOIN)

    1、先创建两个表Group、User,两表的关系是N:N

     1 CREATE TABLE [dbo].[Group](
     2         [Id] [int] IDENTITY(1,1) NOT NULL,
     3         [GroupName] [nvarchar](50) NULL,
     4      CONSTRAINT [PK_Group] PRIMARY KEY CLUSTERED
     5     (
     6         [Id] ASC
     7     )WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
     8     ) ON [PRIMARY]
     9 
    10 CREATE TABLE [dbo].[User](
    11         [Id] [int] IDENTITY(1,1) NOT NULL,
    12         [UserName] [nvarchar](50) NULL,
    13         [GroupId] [int] NULL,
    14      CONSTRAINT [PK_User] PRIMARY KEY CLUSTERED
    15     (
    16         [Id] ASC
    17     )WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
    18     ) ON [PRIMARY]
    建表

    2、测试数据

            

    1 INSERT [dbo].[User] ([Id], [UserName], [GroupId]) VALUES (1, N'张1', 1)
    2 INSERT [dbo].[User] ([Id], [UserName], [GroupId]) VALUES (2, N'张2', 2)
    3 INSERT [dbo].[User] ([Id], [UserName], [GroupId]) VALUES (3, N'张3', 4)
    4 
    5 INSERT [dbo].[Group] ([Id], [GroupName]) VALUES (1, N'A')
    6 INSERT [dbo].[Group] ([Id], [GroupName]) VALUES (2, N'B')
    7 INSERT [dbo].[Group] ([Id], [GroupName]) VALUES (3, N'C')
    插入数据

    3、连接大全

    Sql:
    SELECT [t0].[GroupName], [t1].[UserName]
    FROM [Group] AS [t0]
    INNER JOIN [User] AS [t1] ON ([t0].[Id]) = [t1].[GroupId]
    
    Linq to Sql:
    from g in Groups
    join u in Users
    on g.Id  equals u.GroupId
    select new { GroupName=g.GroupName, UserName=u.UserName}
    
    Lambda:
    Groups.Join 
    (
          Users, 
          g => (Int32?)(g.Id), 
          u => u.GroupId, 
          (g, u) => 
             new  
             {
                GroupName = g.GroupName, 
                UserName = u.UserName
             }
       )
    内连接(INNER JOIN)

    Sql:
    -- Region Parameters
    DECLARE @p0 NVarChar(1000) = ''
    -- EndRegion
    SELECT [t0].[GroupName], 
        (CASE 
            WHEN [t2].[test] IS NULL THEN CONVERT(NVarChar(50),@p0)
            ELSE [t2].[UserName]
         END) AS [UserName]
    FROM [Group] AS [t0]
    LEFT OUTER JOIN (
        SELECT 1 AS [test], [t1].[UserName], [t1].[GroupId]
        FROM [User] AS [t1]
        ) AS [t2] ON ([t0].[Id]) = [t2].[GroupId]
    
    Linq to Sql:
    from g in Groups
    join u in Users
    on g.Id  equals u.GroupId
    into Grp
    from grp in Grp.DefaultIfEmpty()
    select new { GroupName=g.GroupName, UserName=(grp==null)?"":grp.UserName}
    
    Lambda:
    Groups.GroupJoin (
          Users, 
          g => (Int32?)(g.Id), 
          u => u.GroupId, 
          (g, Grp) => 
             new  
             {
                g = g, 
                Grp = Grp
             }
       ) .SelectMany (
          temp0 => temp0.Grp.DefaultIfEmpty (), 
          (temp0, grp) => 
             new  
             {
                GroupName = temp0.g.GroupName, 
                UserName = (grp == null) ? "" : grp.UserName
             }
       )
    左连接(LEFT JOIN)

    Sql:
    -- Region Parameters
    DECLARE @p0 NVarChar(1000) = ''
    -- EndRegion
    SELECT 
        (CASE 
            WHEN [t2].[test] IS NULL THEN CONVERT(NVarChar(50),@p0)
            ELSE [t2].[GroupName]
         END) AS [GroupName], [t0].[UserName]
    FROM [User] AS [t0]
    LEFT OUTER JOIN (
        SELECT 1 AS [test], [t1].[Id], [t1].[GroupName]
        FROM [Group] AS [t1]
        ) AS [t2] ON [t0].[GroupId] = ([t2].[Id])
    
    Linq to Sql:
    from u in Users
    join g in Groups
    on u.GroupId equals g.Id
    into Grp
    from grp in Grp.DefaultIfEmpty()
    select new { GroupName=(grp==null)?"":grp.GroupName, UserName=u.UserName}
    
    Lambda:
    Users.GroupJoin (
          Groups, 
          u => u.GroupId, 
          g => (Int32?)(g.Id), 
          (u, Grp) => 
             new  
             {
                u = u, 
                Grp = Grp
             }
       ).SelectMany (
          temp0 => temp0.Grp.DefaultIfEmpty (), 
          (temp0, grp) => 
             new  
             {
                GroupName = (grp == null) ? "" : grp.GroupName, 
                UserName = temp0.u.UserName
             }
       )
    右连接(RIGHT JOIN)

    Sql:
    -- Region Parameters
    DECLARE @p0 NVarChar(1000) = ''
    DECLARE @p1 NVarChar(1000) = ''
    -- EndRegion
    SELECT DISTINCT [t7].[GroupName], [t7].[value] AS [UserName]
    FROM (
        SELECT [t6].[GroupName], [t6].[value]
        FROM (
            SELECT [t0].[GroupName], 
                (CASE 
                    WHEN [t2].[test] IS NULL THEN CONVERT(NVarChar(50),@p0)
                    ELSE [t2].[UserName]
                 END) AS [value]
            FROM [Group] AS [t0]
            LEFT OUTER JOIN (
                SELECT 1 AS [test], [t1].[UserName], [t1].[GroupId]
                FROM [User] AS [t1]
                ) AS [t2] ON ([t0].[Id]) = [t2].[GroupId]
            UNION ALL
            SELECT 
                (CASE 
                    WHEN [t5].[test] IS NULL THEN CONVERT(NVarChar(50),@p1)
                    ELSE [t5].[GroupName]
                 END) AS [value], [t3].[UserName]
            FROM [User] AS [t3]
            LEFT OUTER JOIN (
                SELECT 1 AS [test], [t4].[Id], [t4].[GroupName]
                FROM [Group] AS [t4]
                ) AS [t5] ON [t3].[GroupId] = ([t5].[Id])
            ) AS [t6]
        ) AS [t7]
    
    Linq to Sql:
    var a=from g in Groups
    join u in Users
    on g.Id  equals u.GroupId
    into Grp
    from grp in Grp.DefaultIfEmpty()
    select new { GroupName=g.GroupName, UserName=(grp==null)?"":grp.UserName};
    
    var b=from u in Users
    join g in Groups
    on u.GroupId equals g.Id
    into Grp
    from grp in Grp.DefaultIfEmpty()
    select new { GroupName=(grp==null)?"":grp.GroupName, UserName=u.UserName};
    
    var c=a.Concat(b).Distinct();
    c.Dump();
    
    Lambda:
    Groups
       .GroupJoin (
          Users, 
          g => (Int32?)(g.Id), 
          u => u.GroupId, 
          (g, Grp) => 
             new  
             {
                g = g, 
                Grp = Grp
             }
       )
       .SelectMany (
          temp0 => temp0.Grp.DefaultIfEmpty (), 
          (temp0, grp) => 
             new  
             {
                GroupName = temp0.g.GroupName, 
                UserName = (grp == null) ? "" : grp.UserName
             }
       )
       .Concat (
          Users
             .GroupJoin (
                Groups, 
                u => u.GroupId, 
                g => (Int32?)(g.Id), 
                (u, Grp) => 
                   new  
                   {
                      u = u, 
                      Grp = Grp
                   }
             )
             .SelectMany (
                temp2 => temp2.Grp.DefaultIfEmpty (), 
                (temp2, grp) => 
                   new  
                   {
                      GroupName = (grp == null) ? "" : grp.GroupName, 
                      UserName = temp2.u.UserName
                   }
             )
       )
       .Distinct ()
    全连接(FULL OUTER JOIN)

    Sql:
    SELECT [t0].[GroupName], [t1].[UserName]
    FROM [Group] AS [t0], [User] AS [t1]
    
    Linq to Sql:
    from g in Groups
    from u in Users
    select new { GroupName=g.GroupName, UserName=u.UserName}
    
    Lambda:
    Groups.SelectMany 
    (
          g => Users, 
          (g, u) => 
             new  
             {
                GroupName = g.GroupName, 
                UserName = u.UserName
             }
       )
    笛卡儿积(CROSS JOIN)

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  • 原文地址:https://www.cnblogs.com/zhaoshujie/p/11484651.html
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