LINQ中连接操作符(九) --两个条件

多条件的情况就是把单个字段改写成匿名类的形式进行等值连接，这里只是列举内连接形式，左/右连接类似，只是在写条件串联的时候需要注意一下

SQL语句写法：

-- SQL语句的写法
select c.Id,c.CategoryName,p.Name "ProductName", p.CreateTime "PublishTime" from listCategory c,listProduct p
where c.Id = p.Id and c.CategoryId = p.CategoryId

Sample Code：

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

namespace ConnectOperation
{
    public class Category
    {
        public int Id { get; set; }
        public int CategoryId { get; set; }
        public string CategoryName { get; set; }
        public DateTime CreateTime { get; set; }
    }

    public class Product
    {
        public int Id { get; set; }
        public int CategoryId { get; set; }
        public string Name { get; set; }
        public double Price { get; set; }
        public DateTime CreateTime { get; set; }
    }

    class Program
    {
        static void Main(string[] args)
        {
            // 初始化数据
            List<Category> listCategory = new List<Category>()
            {
              new Category(){ Id=1,CategoryId=1,CategoryName="计算机",CreateTime=DateTime.Now.AddYears(-1)},
              new Category(){ Id=2,CategoryId=1,CategoryName="文学",CreateTime=DateTime.Now.AddYears(-2)},
              new Category(){ Id=3,CategoryId=2,CategoryName="高校教材",CreateTime=DateTime.Now.AddMonths(-34)},
              new Category(){ Id=4,CategoryId=3,CategoryName="心理学",CreateTime=DateTime.Now.AddMonths(-34)}
            };
            List<Product> listProduct = new List<Product>()
            {
               new Product(){Id=1,CategoryId=1, Name="C#高级编程第10版", Price=100.67,CreateTime=DateTime.Now},
               new Product(){Id=2,CategoryId=1, Name="Redis开发和运维", Price=69.9,CreateTime=DateTime.Now.AddDays(-19)},
               new Product(){Id=3,CategoryId=3, Name="活着", Price=57,CreateTime=DateTime.Now.AddMonths(-3)},
               new Product(){Id=4,CategoryId=2, Name="高等数学", Price=97,CreateTime=DateTime.Now.AddMonths(-1)},
               new Product(){Id=5,CategoryId=6, Name="国家宝藏", Price=52.8,CreateTime=DateTime.Now.AddMonths(-1)}
            };

            // 1、查询表达式
            var queryExpress1 = from c in listCategory
                                join p in listProduct on new { c.Id, c.CategoryId } equals new { p.Id, p.CategoryId }
                                select new { Id = c.Id, CategoryName = c.CategoryName, ProductName = p.Name, PublishTime = p.CreateTime };
            Console.WriteLine("查询表达式输出写法一:");
            foreach (var item in queryExpress1)
            {
                Console.WriteLine($"id:{item.Id},CategoryName:{item.CategoryName},ProductName:{item.ProductName},PublishTime:{item.PublishTime}");
            }
            var queryExpress2 = from c in listCategory
                                from p in listProduct
                                where c.Id.Equals(p.Id) && c.CategoryId.Equals(p.CategoryId)
                                select new { Id = c.Id, CategoryName = c.CategoryName, ProductName = p.Name, PublishTime = p.CreateTime };
            Console.WriteLine("查询表达式输出写法二:");
            foreach (var item in queryExpress2)
            {
                Console.WriteLine($"id:{item.Id},CategoryName:{item.CategoryName},ProductName:{item.ProductName},PublishTime:{item.PublishTime}");
            }

            // 2、方法语法
            Console.WriteLine("方法语法输出:");
            var queryFun = listCategory.Join(listProduct, c => new { c.Id, c.CategoryId }, p => new { p.Id, p.CategoryId }, (c, p) => new { Id = c.Id, CategoryName = c.CategoryName, ProductName = p.Name, PublishTime = p.CreateTime });
            foreach (var item in queryFun)
            {
                Console.WriteLine($"id:{item.Id},CategoryName:{item.CategoryName},ProductName:{item.ProductName},PublishTime:{item.PublishTime}");
            }

            Console.ReadKey();
        }
    }
}