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
            };

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]

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
             };

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]

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
             };

    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]