LINQ基础篇（上）

LINQ  

Language Integrated Query 语言集成查询

 

举个例子

            int[] scores = new int[] { 65,50,45,90,87,65};
            Console.WriteLine("高分有：");
            //方法一
            var highscore = from score in scores
                            where score > 80
                            select score;
            var highscore2 = scores.Where(score => score > 80);
            foreach (var item in highscore)
            {
                Console.WriteLine(item);
            }
            //方法二
            for (int i = 0; i < scores.Length; i++)
            {
                if (scores[i]>80)
                {
                    Console.WriteLine(scores[i]);
                }
            }
            Console.ReadKey();    

代码分析：

1 方法一和方法二都对分数数组进行处理，方法二是一般的用For循环，方法一是查询表达式和方法表达式两种方式的LINQ

2 方法一中的方法表达式 var highscore2 = scores.Where(score => score > 80); 运用了Lambda表达式，形式更为简洁

3 方法一中的查询表达式 var highscore = from score in scores where score > 80 select score;

与正常的SQL语句有一点类似，有关键字select from where等

4 通过例子可以看出LINQ是对数据的处理。

 

关键字讲解

from in

作用：引用数据源，比如数组、集合、数据库的表、DataTable

语法：from 别名 in 数据源

 

where

作用：条件过滤 多个条件时用&& ||

 

Select

作用：表达式需返回的值

语法：单一返回 select 需返回的 多个返回 select new {需返回的A,需返回的B}

 

group by into

作用：根据分类依据对数据进行分类

语法: 单一分类 group 别名A by 别名A.字段 into 别名B

多个分类 group 别名A by new {别名A.字段…} into 别名B

 

orderby ascending descending

作用：查询结果排序

 

Join on equals into

作用：数据源连接,左右连接，内连接

语法：from 别名1 in 数据源1 join 别名2 in 数据源2 on 别名1.字段 equals 别名2.字段 into 别名3

 

代码例子

定义Student类

public class Student
    {
        public string StudentNo { get; set; }
        public string Name { get; set; }
        public int Age { get; set; }
        public string Sex { get; set; }
        public string ClassNo { get; set; }
    }

定义ClassInfo类

 class ClassInfo
    {
        public string ClassNo { get; set; }
        public string ClassName { get; set; }
        public string HeadTeacher { get; set; }
    }

调用方式

 List<Student> listStudent = new List<Student>
            {
                new Student { StudentNo="1",Name="张三",Age=18,Sex="男",ClassNo="1" },
                new Student { StudentNo="2",Name="李四",Age=19,Sex="男",ClassNo="1" },
                new Student { StudentNo="3",Name="王五",Age=18,Sex="男",ClassNo="1" },
                new Student { StudentNo="4",Name="张珊",Age=17,Sex="女",ClassNo="2" },
                new Student { StudentNo="5",Name="王小明",Age=20,Sex="男",ClassNo="1" },
                new Student { StudentNo="6",Name="刘晓",Age=18,Sex="女",ClassNo="1" },
                new Student { StudentNo="7",Name="赵尚武",Age=19,Sex="女",ClassNo="1" },
                new Student { StudentNo="8",Name="关翊",Age=20,Sex="女",ClassNo="2" },
                new Student { StudentNo="9",Name="吴才",Age=17,Sex="男",ClassNo="2" },

            };
            List<ClassInfo> listClassInfo = new List<ClassInfo>
            {
                new ClassInfo { ClassNo="1",ClassName="育才一班",HeadTeacher="王一晟" },
                new ClassInfo { ClassNo="2",ClassName="育才二班",HeadTeacher="张天天"},
            };
            var p = from s in listStudent
                    group s by new { s.ClassNo, s.Sex } into g
                    join c in listClassInfo on g.Key.ClassNo equals c.ClassNo
                    orderby g.Count() descending, g.Key.Sex descending
                    select new { sex=g.Key.Sex, classname=c.ClassName,headteacher=c.HeadTeacher, Count = g.Count() };
            var lp = listStudent.GroupBy(s => new { s.ClassNo, s.Sex})
                    .Join(listClassInfo, g => g.Key.ClassNo, c => c.ClassNo, (g, c) => new { c.ClassName, c.HeadTeacher, g.Key.Sex,count=g.Count() })
                    .OrderByDescending(h =>h.count)
                    .ThenByDescending(h => h.Sex)
                    .Select(h=>new { sex = h.Sex, classname = h.ClassName, headteacher = h.HeadTeacher, Count = h.count });
            foreach (var item in lp)
            {
                Console.WriteLine("班级：{0},班主任：{1}，性别：{2}，人数{3}", item.classname,item.headteacher, item.sex, item.Count);
            }
            Console.ReadKey();

输出

注意 此处group by应该在join之前，与SQL语句的刚好相反，原因是先join再group by导

致listClassInfo中的字段无法访问，需要嵌套foreach来输出