EntityFramework 使用Linq处理内连接(inner join)、外链接(left/right outer join)、多表查询

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场景：在实际的项目中使用EntityFramework都会遇到使用Ef处理连接查询的问题，这里做一些小例子如何通过Linq语法处理内连接(inner join)、外连接(left/right outer join)；

废话不多说先看实体类：

1.内连接：

Linq：

var query = from st in context.SchoolBoys            join gl in context.SchoolGirls on st.GirlfriendId equals gl.Id            select new            {                Id = st.Id,                Name = st.Name,                GirlfriendName = gl.Name            };

sql：

SELECT     1 AS [C1],     [Extent1].[Id] AS [Id],     [Extent1].[Name] AS [Name],     [Extent2].[Name] AS [Name1]    FROM  [dbo].[SchoolBoys] AS [Extent1]    INNER JOIN [dbo].[SchoolGirls] AS [Extent2] ON [Extent1].[GirlfriendId] = [Extent2].[Id]

2.外连接(这里只介绍左外连接)

Linq:

var  query = from st in context.SchoolBoys             join cl in context.Classes on st.ClassId equals cl.Id into cls             from c in cls.DefaultIfEmpty()             select new             {                 Id = st.Id,                 Name = st.Name,                 ClassName = c.Name             };

sql：

SELECT     1 AS [C1],     [Extent1].[Id] AS [Id],     [Extent1].[Name] AS [Name],     [Extent2].[Name] AS [Name1]    FROM  [dbo].[SchoolBoys] AS [Extent1]    LEFT OUTER JOIN [dbo].[Classes] AS [Extent2] ON [Extent1].[ClassId] = [Extent2].[Id]

3.多表混合

linq:

var query3 = from st in context.SchoolBoys             join gl in context.SchoolGirls on st.GirlfriendId equals gl.Id into sgs             from sg in sgs             join cl in context.Classes on sg.ClassId equals cl.Id into cls             from cla in cls.DefaultIfEmpty()             join g in context.Grades on cla.GradeId equals g.Id into gs             from gr in gs.DefaultIfEmpty()             select new             {                  Id = st.Id,                  Name = st.Name,                  ClassName = cla.Name,                  GradeName = gr.Name             };

sql：

    SELECT     1 AS [C1],     [Extent1].[Id] AS [Id],     [Extent1].[Name] AS [Name],     [Extent3].[Name] AS [Name1],     [Extent4].[Name] AS [Name2]    FROM    [dbo].[SchoolBoys] AS [Extent1]    INNER JOIN [dbo].[SchoolGirls] AS [Extent2] ON [Extent1].[GirlfriendId] = [Extent2].[Id]    LEFT OUTER JOIN [dbo].[Classes] AS [Extent3] ON [Extent2].[ClassId] = [Extent3].[Id]    LEFT OUTER JOIN [dbo].[Grades] AS [Extent4] ON [Extent3].[GradeId] = [Extent4].[Id]

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