linq 之左连接

最新推荐文章于 2025-07-15 14:23:01 发布

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最新推荐文章于 2025-07-15 14:23:01 发布

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文章标签：面试

原文链接：http://www.cnblogs.com/ushou/p/3408014.html

博客展示了用户表与文章表的多种连接查询代码，包括用户表左连接文章表、文章表左连接用户表以及用户表连接文章表，同时指出部分写法实际相当于连接，需避免，代码示例具有一定参考价值。

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List<ArticleModel> articleList = articleRepository.GetAllArticle();
List<UsersModel> userList = usersRepository.GetAllUsers();

//用户表左连接文章表
var usersLeftJoin = (from u in userList
join a in articleList
on u.Id equals a.Author into users
from us in users.DefaultIfEmpty()
select new
{
author = us == null ? "" : us.Author.ToString(),
name = u.Name.Trim()
}).ToList();

//要避免这种写法，这种写法查出来的数据实际上相当于连接
var usersLeftJoin1 = (from u in userList
join a in articleList
on u.Id equals a.Author into users
from us in users
select new
{
author = us.Author,
name = u.Name.Trim()
}).ToList();

//文章表左连接用户表
var articleLeftJoin = (from a in articleList
join u in userList
on a.Author equals u.Id into users
from us in users.DefaultIfEmpty()
select new
{
title = a.Title.Trim(),
name = us == null ? "" : us.Name.Trim(),
author = a.Author
}).ToList();

//要避免这种写法，这种写法查出来的数据实际上相当于连接
var articleLeftJoin1 = (from a in articleList
join u in userList
on a.Author equals u.Id into users
from us in users
select new
{
name = us.Name.Trim(),
author = a.Author
}).ToList();

//用户表连接文章表
var result = (from a in articleList
from u in userList
where a.Author == u.Id
select new
{
name = u.Name,
author = a.Author
}).ToList();

转载于:https://www.cnblogs.com/ushou/p/3408014.html