zoukankan      html  css  js  c++  java
  • Linq中使用Left Join

      准备一些测试数据,如下: 

    复制代码
    use Test
    Create table Student(
    ID int identity(1,1) primary key,
    [Name] nvarchar(50) not null
    )

    Create Table Book(
    ID int identity(1,1) primary key,
    [Name] nvarchar(50)not null,
    StudentID int not null
    )

    insert into Student values('张三')
    insert into Student values('李四')
    insert into Student values('王五')
    select * from student

    --张三借的书
    insert into Book values('红楼',1)
    insert into Book values('大话红楼',1)

    --李四借的书
    insert into Book values('三国',2)

    --王五没借书

    --一本错误的记录
    insert into Book values('错误时怎样练成的',111)

    --左连接
    select s.name,b.name from student as s
    left join Book as b on s.id=b.studentid

    --右连接
    select s.name,b.name from student as s
    right join Book as b on s.id=b.studentid
    复制代码

     要用Linq实现左连接,写法如下 

    复制代码
    DataClasses1DataContext db = new DataClasses1DataContext();
    var leftJoinSql = from student in db.Student
    join book in db.Book on student.ID equals book.StudentID into temp
    from tt in temp.DefaultIfEmpty()
    select new
    {
    sname= student.Name,
    bname = tt==null?"":tt.Name//这里主要第二个集合有可能为空。需要判断
    };
    复制代码

     用Linq实现右连接,写法如下

    复制代码
    DataClasses1DataContext db=new DataClasses1DataContext();
    var rightJoinSql = from book in db.Book
    join stu in db.Student on book.StudentID equals stu.ID into joinTemp
    from tmp in joinTemp.DefaultIfEmpty()
    select new {
    sname=tmp==null?"":tmp.Name,
    bname=book.Name

    };
    复制代码

     原文地址:http://www.cnblogs.com/xinjian/archive/2010/11/17/1879959.html 

  • 相关阅读:
    学习java随笔第五篇:流程控制
    学习java随笔第四篇:运算符
    学习java随笔第三篇:java的基本数据类型
    知识精简
    性能优化(详细)
    2,8,10,16进制转换
    前端优化35例
    性能优化
    字面量自定义事件

  • 原文地址:https://www.cnblogs.com/gusongbanyue/p/4741533.html
Copyright © 2011-2022 走看看