EF 左连接的试验

最新推荐文章于 2025-06-17 19:53:03 发布

原创最新推荐文章于 2025-06-17 19:53:03 发布 · 386 阅读

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文章标签：

#windows #c# #开发语言 #left join

1.概要

左连接是很常用的方法。

2.代码

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace 表对象化链表排序
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine();
            Console.WriteLine("表对象化链表排序");
            Program p = new Program();
            p.main();
            Console.ReadKey();
        }
        private void main()
        {
            test_groupJoin();
            //test_join();
            //test_leftJoin();
            //test2();
        }
        private void test_groupJoin()
        {
            var groupList = A.getList().GroupJoin(B.getList(), a => a.bid, b => b.id, (a, b) => new { a.ns, a.id, b });
            foreach (var a in groupList)
            {
                Console.WriteLine(a.id + " " + a.ns + " ");
                var c = a.b;
                var d = c.Select(h => new { h.ns2 }).FirstOrDefault();
                if (d != null)
                {
                    Console.WriteLine(d.ns2);
                }
                Console.WriteLine();
            }
        }
        private void test_join()
        {
            var tlist = from a in A.getList() join b in B.getList() on a.bid equals b.id select new { a.id,a.ns,bid= b.id,b.ns2};
            foreach(var it in tlist)
            {
                Console.WriteLine(it.id + " " + it.ns);
            }
        }
        private void test_leftJoin()
        {
            var tlist = from a in A.getList() join b in B.getList() on a.bid equals b.id 
                        into dd 
                        from dt in dd.DefaultIfEmpty()
                        select new { a.id, a.ns, bid = dt==null?0:dt.id, ns2 = dt==null?"":dt.ns2};
            foreach (var it in tlist)
            {
                Console.WriteLine(it.id + " " + it.ns+ " "+it.ns2);
            }
        }
        private void test2()
        {
            List<List<int>> numbers = new List<List<int>>(){
                new List<int>{1,2,3},
                new List<int>{4,5,6},
                new List<int>{7,8,9} 
            };
            var result = numbers.SelectMany(collection => collection);
            foreach (var item in result)
            {
                Console.WriteLine(item);
            }
        }
    }
    public class A
    {
        public int id;
        public string ns;
        public int bid;
        public A(int id,string n,int bid) {
            this.id = id;
            this.ns = n;
            this.bid = bid;
        }
        public static List<A> getList()
        {
            List<A> li = new List<A>();
            li.Add(new A(1, "a1",1));
            li.Add(new A(2, "a2",2));
            li.Add(new A(3, "a3",0));
            return li;
        }
    }
    public class B
    {
        public int id;
        public string ns2;
        public B(int id,string n)
        {
            this.id = id;
            this.ns2 = n;
        }
        public static List<B> getList()
        {
            List<B> li = new List<B>();
            li.Add(new B(1, "b1"));
            li.Add(new B(2, "b2"));
            return li;
        }
    }

}

3.运行结果

3.1 test_groupJoin();

表对象化链表排序
1 a1
b1

2 a2
b2

3 a3

3.2 test_join

表对象化链表排序
1 a1
2 a2

3.3 test_leftJoin();

表对象化链表排序
1 a1 b1
2 a2 b2
3 a3

4.参考连接

如何在EF中实现left join（左联接）查询_ef left join_菜鸟de成长历程的博客-优快云博客

ef 使用left join的两种写法_ef left join_看月亮的向月葵的博客-优快云博客

EF 实现left join左联接查询_51CTO博客_querywrapper left join

https://www.cnblogs.com/keeplearningandsharing/p/16625086.html

https://www.cnblogs.com/Kit-L/p/14839434.html