如何写LEFT JOIN和LINQ [英] How to write LEFT JOIN ith LINQ

问题描述

你好，



我有3张桌子



参赛者 - 身份证，姓名，类别

比赛 - 同上，名字

参赛者竞赛 - 比赛，比赛，参赛者



如何编写c＃LINQ for以下查询？

  SELECT  Contst.Name，Cat.Category 
  FROM 参赛者Contst 
  LEFT   JOIN 参赛者竞赛ContstComp 
  ON  Contst.Id = ContstComp.Contestant4 
  JOIN 类别Cat 
  ON  Contst.Category = Cat.Id 
  WHERE  ContstComp.Competition  IS   NULL  

提前致谢

解决方案

  var  query = 来自比赛 in 参赛者
 加入 ContstComp   $ b $上的参赛者竞赛b contest.ID等于ContstComp.Contestant4  into  ContestContestComp 
  from  ccmp  in  ContestContestComp.DefaultIfEmpty（）
  join  Cat  类别on contest.Category等于Cat.Id 
 其中 ContstComp.Competition ==  null  
 选择  new  {Contst.Name，Cat.Category} 

加入< identifier>作为对加入结果的引用。它就像SQL中的'AS'关键字一样。在上面的查询中，连接结果集称为ContestContestComp。

DefaultIfEmpty - 如果集合为空，则返回默认实例。



我认为你推荐这篇文章会更好，

http://msdn.microsoft.com/en-us/library/vstudio/bb397895.aspx [ ^ ]

定义LINQ查询在这种情况下，您应该发布用于查询的类。同时，请查看如何：执行左外连接（C＃编程指南）中的例外情况 [ ^ ]

Hi there,

I have 3 tables

Contestant - Id, Name, Category
Competition - Id, Name
ContestantsInCompetition - Id, Competition, Contestant

How can I write c# LINQ for the following Query?

SELECT		Contst.Name,	Cat.Category	
FROM		Contestant Contst
LEFT JOIN	ContestantsInCompetition ContstComp
ON	        Contst.Id = ContstComp.Contestant4
JOIN		Category Cat
ON		Contst.Category = Cat.Id
WHERE		ContstComp.Competition IS NULL

Thanks in advance

解决方案

var query=from contest in Contestant 
	join ContstComp in ContestantsInCompetition on 
	contest.ID equals ContstComp.Contestant4 into ContestContestComp
	from ccmp in ContestContestComp.DefaultIfEmpty()
	join Cat in Category on contest.Category equals Cat.Id
	where ContstComp.Competition == null
	select new {Contst.Name,Cat.Category}

"Into <identifier>" serve as a reference to the results of join. It is just like ‘AS" keyword in SQL. In above query, join result set referred as "ContestContestComp".
"DefaultIfEmpty" - returns a default instance if the collection is empty.

I think it is better you refer this article,
http://msdn.microsoft.com/en-us/library/vstudio/bb397895.aspx[^]

To define the LINQ query for your case, you should post the classes which you use for the query. Meanwhile, have a look at the exmaple in How to: Perform Left Outer Joins (C# Programming Guide)[^]

这篇关于如何写LEFT JOIN和LINQ的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！