Linq无聊练习系列4--join练习

/**************join 练习*******************/
            //对于1对多关系
            var list =from c in ctx.T_Student
                      from s in c.T_Score
                      where c.stuName=="黄阳"
                      select s;
            //也可以间接的通过表关联其它表，这个结果和上边的查询结果相同
            var list1 = from s in ctx.T_Score
                        where s.T_Student.stuName=="黄阳"
                        select s;
            //多对多关系,他们之间的关系是1：M:1
            var list2 = from s in ctx.T_Cource
                        from c in s.T_Score
                        select new {
                            s.courceName,
                            c.score,
                            c.T_Student.stuName
                        };
            //如果表里边有隶属关系，可以使用自联结，由于表的设计不是很好，所以没有包括这种情况
            //双向链接
            var list3 = from s in ctx.T_Student
                        join t in ctx.T_Score
                        on s.stuNumber
                        equals t.stuNumber
                        into g
                        select new {
                           stuName =  s.stuName,
                           score = g.Sum(k=>k.score)               
                        };
            //三向链接
            var list4 = from s in ctx.T_Score
                        join c in ctx.T_Cource
                        on s.courceNumber
                        equals c.courceNumber into temp1
                        join t in ctx.T_Student
                        on s.stuNumber
                        equals t.stuNumber into temp2
                        select new {
                           Score = s.score,
                           Name = temp1.Count()
                        };
            //左外部链接
            var list5 = from s in ctx.T_Student
                        join c in ctx.T_Score
                        on s.stuNumber
                        equals c.stuNumber
                        into temp1
                        from k in temp1.DefaultIfEmpty()
                        select new
                        {
                            stuName = s.stuName,
                            sumScore = k.score
                        };
            //投影的let操作
            var list6 = from s in ctx.T_Student
                        join c in ctx.T_Score
                        on s.stuNumber
                        equals c.stuNumber into temp1
                        let studentInfo = s.stuName +s.stuSex
                        from k in temp1
                        select new {
                            stuNumber=s.stuNumber,
                            CoreOfStudents = k.score,
                            studentInfomation = studentInfo
                        };
                //使用组合健来表示多表之间的关系 ,这个由于表中设计时候，基本只有一个主键，没有用到联合主键  
            //一侧可为空另一侧不能为空的链接
            var list7 = from s in ctx.T_Student
                        join c in ctx.T_Score
                        on Convert.ToInt32(s.stuNumber)
                        equals (int ?)Convert.ToInt32(c.stuNumber) into temp1
                        select new {
                            stuName = s.stuNumber,
                            sumCore = temp1.Sum(f=>f.score)
                        };