c#中的linq二

c#中的linq二

 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace LinqInfo
{
    public class Stu
    {
        public int ID { set; get; }

        public string Name { get; set; }

        public string Tel { get; set; }

        public string Address { get; set; }
    }

    public class GradeInfo
    {
        public int StuID {get;set; }
        public string Course{get;set;}
        public int grade{get;set;}

    }
    class Program
    {
        static void Main(string[] args)
        {
            List<Stu> list = new List<Stu>()
            {
                new Stu{ID=1,Name="jack1",Tel="110",Address="ChengDu"},
                new Stu{ID=2,Name="jack2",Tel="112",Address="ChengDu"},
                new Stu{ID=3,Name="jack3",Tel="113",Address="ChengDu"},
                new Stu{ID=4,Name="jack4",Tel="114",Address="ChengDu"},
                new Stu{ID=5,Name="jack5",Tel="115",Address="ChengDu"}

            };

            //where 关键字
            var query =
                 from li in list
                 where li.Name == "jack1"
                 select li;
            //order by关键字的 
            //ascending  升序
            //descending 降序
            var query1 =
                from s in list
                orderby s.ID ascending
                select s;
            //group by的使用
            var query2 =
                from s in list
                group s by s.Address into g
                select g;
            //group by 求最大值
            var max =
                from s in list
                group s by s.Address into g
                select new { 
                  g.Key,      //分组字段
                  MaxId=g.Max(p=>p.ID) //最大的ID
                };

            //group by 求最小值
            var min =
                from s in list
                group s by s.Address into g
                select new {
                 g.Key,
                 MidId=g.Min(p=>p.ID)
                };

            //group by 平均值
            var avg =
                from s in list
                group s by s.Address into g
                select new
                {
                    g.Key,
                    avgInfo = g.Average(p => p.ID)
                };
            //group by 求和
            var sum =
                from s in list
                group s by s.Address into g
                select new
                {
                    g.Key,
                    Sum=g.Sum(p=>p.ID)
                };
            //这些就是group by 的基本使用滴呀；


            //接下来就是各种连接滴呀;
            List<GradeInfo> gradeList = new List<GradeInfo>()
            {
                 new GradeInfo(){StuID=1,Course="语文",grade=100},
                 new GradeInfo(){StuID=1,Course="英语",grade=99},
                 new GradeInfo(){StuID=1,Course="数学",grade=88},
                 new GradeInfo(){StuID=2,Course="语文",grade=66},
                 new GradeInfo(){StuID=2,Course="英语",grade=90},
                 new GradeInfo(){StuID=2,Course="数学",grade=67},
                 //模拟错误数据
                 new GradeInfo(){StuID=110,Course="wrong",grade=120}
            };


            //然后再来我们的各种连接查询滴呀;
            var innerJoin =
                from s in list
                join g in gradeList
                on s.ID equals g.StuID
                select new
                {
                   Name=s.Name,
                   Grade=g.grade
                };


            //左连接;
            var leftJoin =
                from s in list
                join g in gradeList
                on s.ID equals g.StuID into joinData
                from g in joinData.DefaultIfEmpty()
                select new
                {
                    Name=s.Name,
                    Grade= g!=null ? g.grade:0 //返回相同的数据类型；
                };

              //整体来说，再数据赛选上的效果是挺不错滴呀;
            
              //右连接； 其实左连接 和右 连接是一个相对的概念;
            var rightJoin =
                from g in  gradeList
                join s in list
                on  g.StuID equals s.ID into joinData
                from s in joinData.DefaultIfEmpty()
                select new
                {
                    Grade=g.grade,
                    Name=s!=null?s.Name:""
                };

            var result = rightJoin;
            //最后在附带一中linq分页的方法；
            var val = (from u in list
                       orderby u.ID ascending
                       select u).Skip(2).Take(2);
            //每页两条，第二页
            /*
             * 
             * var q=query.take(n) 这样只能取出前条n记录，
             * 但是还是让我们看到了分页的曙光，还是还有个skip吗，
             * skip可以取出除去满足条件的剩下的记录。
             * 如果我们要把数据以每页pagesize的数量来进行分页，
             * 很简单，很skip(pagersize*pagenum)然后在剩下的记录中take(pagesize)不就得到了想要的数据了么。
             * 
             * */
            var xx = val;

            Console.WriteLine("over");
            Console.ReadLine();
        }


    }
}