Asp.net网站开发（一）LINQ TO SQL 之八大字句

1.   from  in子句：指定查询操作的数据源和变量范围

//创建LinqDB数据库上下文实例

           DataClasses1DataContext db = new DataClasses1DataContext();

 //使用lINQ查询表达式查询数据

           var query = from stu in db.student

                       join   m  in db.mark  on  stu.sid equals m.sid

                       select new { 姓名=stu.sname,科目=m.subject,成绩=m.score };

           //设置数据源并绑定数据

          GridView1.DataSource = query;

          GridView1.DataBind();

 

2.   select子句：指定查询结果的类型和表现形式

3.   where子句：删选元素的逻辑条件，一般由逻辑运算符组成

4.   groupby子句：对查询进行分组

  DataClasses1DataContext db = new DataClasses1DataContext();  
  
            var query = from m in db.mark

                        group m by m.mid into g

                        orderby g.Key

                        select new { g.Key, 成绩 = g.Max(c => c.score) };

         
           GridView1.DataSource = query;  
  
           GridView1.DataBind();  

5.   orderby子句：对查询结果排序，可以为升序或降序

降序：

DataClasses1DataContext db = new DataClasses1DataContext();
             var q = db.mark
                 .OrderByDescending(P => P.score )
                 .ToList();
             GridView1.DataSource = q;

             GridView1.DataBind();

或者：

升序：

//创建LinqDB数据库上下文实例

           DataClasses1DataContext db = new DataClasses1DataContext();

            //使用lINQ查询表达式查询数据

           var  query = from m in  db.mark

                       join  stu in db.student on  m.sid equals stu.sid

                       orderby  m.score  ascending

                       select  new { 姓名 =stu.sname, 科目 = m.subject, 成绩 = m.score };

 

            //设置数据源并绑定数据

           GridView1.DataSource = query;

           GridView1.DataBind();

或者

 DataClasses1DataContext db = new DataClasses1DataContext();
             var q = db.mark
                 .OrderBy(P => P.score )
                 .ToList();
             GridView1.DataSource = q;

             GridView1.DataBind();
                 

6.   join子句：连接多个查询操作的数据源

//创建LinqDB数据库上下文实例

           DataClasses1DataContext db = new DataClasses1DataContext();

           //使用lINQ查询表达式查询数据

           var query = from stu in db.student

                       join m in db.mark on stu.sid equals m.sid

                       select  new { 姓名=stu.sname,科目=m.subject,成绩=m.score };

           //设置数据源并绑定数据

          GridView1.DataSource = query;

          GridView1.DataBind();

7.   let子句：引入用于储存查询表达式中的子表达式结果的范围变量

             DataClasses1DataContext db = new DataClasses1DataContext();
             var q = from m in db.mark
                     let n = m.score 
                     where n >90
                     select  m ;
                     
      
             GridView1.DataSource = q;

             GridView1.DataBind();

8.   into子句：提供一个临时标志符，充当对group，select，join，子句的结果

用于select：

             DataClasses1DataContext db = new DataClasses1DataContext();
             var q = from stu in db.student
                     where stu.sid > 2
                     select new { 学号 = stu.sid, 姓名 = stu.sname }
                         into s
                         orderby s.学号
                         select s;
      
             GridView1.DataSource = q;

             GridView1.DataBind();

附数据库代码：

 use master

       go

       if exists(select * from sys.databases where name='Student')

      drop database Student

      create database Student

       go

      use Student

       if exists(select * from sys.tables  where name='student')

     drop database student

     create table student

     (

     sid int primary key identity(1,1) ,

     sname varchar(10) not null

      

     )

     if exists(select * from sys.tables where name='mark')

    drop database mark

     create table mark

     (

     mid int  primary key identity(1,1) ,

     subject varchar(10) not null,

     score int not null,

     sid int references student(sid)

     )

     insert into student values ('林彦俊')

     insert into student values ('灵超')

     insert into student values ('尤长靖')

     insert into student values ('木子洋')

     insert into mark values ('html',100,1)

     insert into mark values ('sql',90,1)

     insert into mark values ('html',10,2)

     insert into mark values ('sql',60,2)

     insert into mark values ('html',99,3)

     insert into mark values ('sql',91,4)

     select *from student

    select * from mark

数据库运行结果：