Linq中怎么查询多张表

我做  这个  只是为了让我自己  以后好找资料而已

#region Group,Join
            //只有join，没有into，内联(inner join)
            //var sql = from c in sdb.Classic
            //          join s in sdb.StuInfo
            //          on c.classID equals s.classID //注意不能用==
            //          select new
            //          {
            //              c.className,
            //              s.stuName
            //          };
            //Console.WriteLine(sql);

            //var sql = from s in sdb.StuInfo
            //          join m in sdb.StuMarks
            //          on s.stuNo equals m.stuNo
            //          into temp                         //many端重新命名
            //          from t in temp.DefaultIfEmpty()   //左连接，DefaultIfEmpty再次遍历右边，缺考人员的成绩默认为null
            //          select new
            //          {
            //              s.stuName,
            //              we = t.writtenExam == null ? 0 : t.writtenExam,
            //              le = t.LabExam == null ? 0 : t.LabExam
            //          };
            //Console.WriteLine(sql);
            //foreach (var item in sql)
            //{
            //    Console.WriteLine("{0}\t{1}\t{2}",item.stuName,item.we,item.le);
            //}

            //三向联接
            //var sql = from c in sdb.Classic
            //          join s in sdb.StuInfo
            //          on c.classID equals s.classID
            //          join m in sdb.StuMarks
            //          on s.stuNo equals m.stuNo
            //          select new
            //          {
            //              c.className,
            //              s.stuName,
            //              we = m.writtenExam == null ? 0 : m.writtenExam,
            //              m.LabExam
            //          };
            //Console.WriteLine(sql);
            //foreach (var item in sql)
            //{
            //    Console.WriteLine("{0}\t{1}\t{2}\t{3}", item.className, item.stuName, item.we, item.LabExam);
            //}

            //双向连接
            var sql = from c in sdb.Classic
                      join s in sdb.StuInfo
                      on c.classID equals s.classID
                      into stu
                      select new
                      {
                          className = c.className,
                          list=stu.ToList<StuInfo>(),
                          count = stu.Count()
                      };
            Console.WriteLine(sql);
            foreach (var item in sql)
            {
                Console.WriteLine("班级：{0},人数：{1}",item.className,item.count);
                foreach (var s in item.list)
                {
                    Console.WriteLine(s.stuName);
                }
            }
            #endregion