【EF学习笔记05】----------DBContext基础查询

//遍历所有学生 DBSet
using (var db = new Entities())
{
    foreach (var student in db.Student)
    {
        ObjectDumper.Write(student.StudentName);
    }
};

using (var db = new Entities())
{
    //查询所有出生日期在1991年1月1日之后的学生
    var day = Convert.ToDateTime("1991-1-1");
    var students = from v in db.Student where v.Birthday >= day select v;
    foreach (var student in students)
    {
        ObjectDumper.Write(student.StudentName+"-"+student.Birthday);
    }
};

using (var db = new Entities())
{
    var students = from v in db.Student where v.Birthday >= Convert.ToDateTime("1991-1-1") select v;
    foreach (var student in students)
    {
        ObjectDumper.Write(student.StudentName+"-"+student.Birthday);
    }
};

using (var db = new Entities())
{
    var students = from v in db.Student
                    orderby v.Birthday
                    select v;
    foreach (var student in students)
    {
        ObjectDumper.Write(student);
    }
};

var students = from v in db.Student
                orderby v.Birthday descending
                select v;

//find查询单个实体
using (var db = new Entities())
{
    var student = db.Student.Find(1);
    ObjectDumper.Write(student);
};

//Single查询单个实体
using (var db = new Entities())
{
    var student = from v in db.Student
                    where v.StudentName=="张三"
                    select v;
    //single方法 查不到记录 或者 多条记录 都会报错
    ObjectDumper.Write(student.Single());
};

//SingleOrDefault查询单个实体
using (var db = new Entities())
{
    var student = from v in db.Student
                    where v.StudentName == "李四"
                    select v;
    //SingleOrDefault方法 查不到记录返回null ，如果查询到多条记录会报错
    ObjectDumper.Write(student.SingleOrDefault());
};

//遍历所有学生
            using (var db = new Entities())
            {
                var students = from v in db.Student select v;
                foreach (var student in students)
                {
                    ObjectDumper.Write(student);
                }
            };

            Console.WriteLine("==========按照生日排序===========");
            //按照生日排序 升序
            using (var db = new Entities())
            {
                var students = from v in db.Student
                                orderby v.Birthday descending
                                select v;
                foreach (var student in students)
                {
                    ObjectDumper.Write(student);
                }
            };

            Console.WriteLine("==========find查询单个实体===========");
            //find查询单个实体
            using (var db = new Entities())
            {
                var student = db.Student.Find(1);
                ObjectDumper.Write(student);
            };

            Console.WriteLine("==========single查询单个实体===========");
            //Single查询单个实体
            using (var db = new Entities())
            {
                var student = from v in db.Student
                                where v.StudentName=="张三"
                                select v;
                //single方法 查不到记录 或者 多条记录 都会报错
                ObjectDumper.Write(student.Single());
            };

            Console.WriteLine("==========SingleOrDefault查询单个实体===========");
            //SingleOrDefault查询单个实体
            using (var db = new Entities())
            {
                var student = from v in db.Student
                                where v.StudentName == "李四"
                                select v;
                //SingleOrDefault方法 查不到记录返回null ，如果查询到多条记录会报错
                ObjectDumper.Write(student.SingleOrDefault());
            };