Linq to EF 內鏈接和左外鏈接

1、內鏈接

 

 內鏈接與SqL中inner join同樣，即找出兩個序列的交集

 

Model1Container model = new Model1Container();
            //內鏈接
            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);
            }

與上面的內鏈接語句相對應的SQL腳本語言以下所示：

 

 

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、左外鏈接

 

左外鏈接與SqL中left join同樣

 

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時必須將join後的表into到一個新的變量gc中，而後要用gc.DefaultIfEmpty()表示外鏈接。
            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，它可以爲實序列提供一個默認的元素。DefaultIfEmpty使用了泛型中的default關鍵字。default關鍵字對於引用類型將返回null,而對於值類型則返回0。對於結構體類型，則會根據其成員類型將它們相應地初始化爲null(引用類型)或0(值類型)

咱們能夠不使用default關鍵字，但在要DefaultIfEmpty中給定當空時的默認對象值。語句以下：

 

//left join, 爲空時使用默認對象
            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="" }     //設置爲空時的默認值
                                )
                                select new
                                {
                                    ClassID = s.CourseCno,
                                    ClassName = gci.Cname,
                                };

 

與上面的左外鏈接語句相對應的SQL腳本語言以下所示：

 

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]