Linq中连接主要有组连接、内连接、左外连接、交叉连接四种。本文主要讲解没连接和左外连接。

本次使用到的数据实体模型具体的创建方法不再累述。该实体模型中包括Student、Course两个表，他们之间是一对多的关系。

一、内连接

 内连接与SqL中inner join一样，即找出两个序列的交集

           Model1Container model = new Model1Container();
            //内连接
            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);
            }

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

二、左外连接

左外连接与SqL中left join一样

            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，然后要用gc.DefaultIfEmpty()表示外连接。
            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操作符，它能够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)

我们可以不使用default关键字，但在要DefaultIfEmpty中给定当空时的默认对象值。语句如下：

            //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,
                                };

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]

以上是自己学习时的一点见解，不对之处请拍砖指教。

Linq to EF 内连接和左外连接