Linq to EF 内链接和左外链接

1、内链接

 

 内链接与SqL中inner join同样，即找出两个序列的交集

 

Model1Container model = new Model1Container();
            //内链接
            var query = from s in model.Student
                        join c in model.Course on s.CourseCno equals c.Cno
                        where c.Cno == 1
                        select new
                        {
                            ClassID = s.CourseCno,
                            ClassName = c.Cname,
                            Student = new
                            {
                                Name = s.Sname,
                                ID = s.Sno
                            }
                        };
            foreach (var item in query)
            {
                Response.Write("ClassID:" + item.ClassID + "ClassName:" + item.ClassName + "Name:" + item.Student.Name);
            }

与上面的内链接语句相对应的SQL脚本语言以下所示：

 

 

SELECT [t0].[CourseCno] AS [ClassID], [t1].[Cname] AS [ClassName], [t0].[Sname] AS [Name], [t0].[Sno] AS [ID]
FROM [Student] AS [t0]
INNER JOIN [Course] AS [t1] ON [t0].[CourseCno] = [t1].[Cno]
WHERE [t1].[Cno] = @p0

 

 

 

2、左外链接

 

左外链接与SqL中left join同样

 

Model1Container model = new Model1Container();
            var query = from s in model.Student
                        join c in model.Course on s.CourseCno equals c.Cno into gc
                        from gci in gc.DefaultIfEmpty()
                        select new
                        {
                            ClassID = s.CourseCno,
                            ClassName = gci.Cname,
                            Student = new
                            {
                                Name = s.Sname,
                                ID = s.Sno
                            }
                        };
            //Outer join时必须将join后的表into到一个新的变量gc中，而后要用gc.DefaultIfEmpty()表示外链接。
            foreach (var item in query)
            {
                Response.Write("ClassID:" + item.ClassID + "ClassName:" + item.ClassName + "Name:" + item.Student.Name);
            }

 

注：上例中使用了DefaultIfEmpty操做符，它可以为实序列提供一个默认的元素。DefaultIfEmpty使用了泛型中的default关键字。default关键字对于引用类型将返回null,而对于值类型则返回0。对于结构体类型，则会根据其成员类型将它们相应地初始化为null(引用类型)或0(值类型)

咱们能够不使用default关键字，但在要DefaultIfEmpty中给定当空时的默认对象值。语句以下：

 

//left join, 为空时使用默认对象
            var leftJoinQuery = from s in model.Student
                                join c in model.Course
                                on s.CourseCno equals c.Cno into gc
                                from gci in gc.DefaultIfEmpty(
                                new Course { Cname = "",Cperiod="" }     //设置为空时的默认值
                                )
                                select new
                                {
                                    ClassID = s.CourseCno,
                                    ClassName = gci.Cname,
                                };

 

与上面的左外链接语句相对应的SQL脚本语言以下所示：

 

SELECT [t0].[CourseCno] AS [ClassID], [t1].[Cname] AS [ClassName], [t0].[Sname] AS [Name], [t0].[Sno] AS [ID]
FROM [Student] AS [t0]
LEFT OUTER JOIN [Course] AS [t1] ON [t0].[CourseCno] = [t1].[Cno]