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();
}

}
}