ADO.net实体类操作

查询对象

StudentManageDBEntities2 studentListEntity = new StudentManageDBEntities2();
            var studentReturn = studentListEntity.StudentList.SingleOrDefault(s => s.ID == 1630200001);
            if (studentReturn != null)
            {
                Console.WriteLine(studentReturn.Name);
            }
            //使用where()方法查询符合条件的数据
            IQueryable<StudentList> query = studentListEntity.StudentList.Where(s => s.Gender == "男");
            foreach (var item in query)
            {
                Console.WriteLine(item.Name + "  " + item.PhoneNumber);
            }

修改对象

         //想要修改对象先找到对象
        StudentManageDBEntities2 studentEntities = new StudentManageDBEntities2();
        //两种查询方法
        //第一种
        studentEntities.StudentList.FirstOrDefault(s => s.ID == 1630200001);
        //第二种
        var stu = (from s in studentEntities.StudentList
                   where s.ID == 1630200001
                   select s).First<StudentList>();
        Console.WriteLine(stu.Name);

删除对象

         //删除对象
        studentEntities.StudentList.Remove(stu);

Linq数据投影：（返回一个新的对象）

                 var stu = from s in studentEntities.StudentList
                  where s.Gender == "男"
                  //返回的数据映射到一个新的对象stu(stu需要遍历)
                      select new { s.Name, s.StudentAddress };

多表查询

        var stuList = from s in studentEntities.StudentList
                      select new
                      {
                       s.Name,
                       s.StudentAddress,
                       s.ClassList.Classid,
                       s.ClassList.ClassName
                      };

        foreach (var item in stuList)
        {
            Console.WriteLine(
            item.Name+
            item.StudentAddress+
            item.Classid+
            item.ClassName);
        }

也可以使用JOIN语句

        var stuList = from s in studentEntities.StudentList
                      join c in studentEntities.ClassList on s.Classid equals c.Classid
                      select new
                      {
                         s.Name,s.StudentAddress,s.Classid,s.ClassList.ClassName
                      };
                      

        foreach (var item in stuList)
        {
            Console.WriteLine(
            item.Name+
            item.StudentAddress+
            item.Classid+
            item.ClassName);
        }

//也可以进行子查询，如下

        var stuList2 = from s in stuList where s.Classid >= 100 select s;
        foreach (var item in stuList2)
        {
            Console.WriteLine(
            item.Name +
            item.StudentAddress +
            item.Classid +
            item.ClassName);
        }

聚合查询

        var stuList = from s in studentEntities.StudentList
                      select new
                      {
                         cunt = (from t in studentEntities.StudentList
                                   select t.ID).Count()
                      };

        stuList.ToList();
        Console.WriteLine(stuList.First());

数据关联操作： 创建对象的时候，同时为其选择外键

         StudentManageDBEntities3 studentEntities = new StudentManageDBEntities3();
        var student = new StudentList()
        {
            Name = "liFlower",
            Gender = "男",
            Birthday = DateTime.Now,
            IDCard = 220153455616355143,
            Age = 15,
            PhoneNumber = "1235656177",
            StudentAddress = "长春",
            //同时操作外键
            Classid = (from c in studentEntities.ClassList where c.ClassName == "英语班" select c.Classid).FirstOrDefault()
        };