LINQ to SQL语句 Union/Intersect/Except

LINQ to SQL statement Union/Intersect/Except

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

namespace BegVCSharp_23_15_SetOperators
{
    class Customer
    {
        public string ID { get; set; }
        public string City { get; set; }
        public string Country { get; set; }
        public string Region { get; set; }
        public decimal Sales { get; set; }
    }
    class Order
    {
        public string ID { get; set; }
        public decimal Amount { get; set; }
    }
    class Program
    {
        static void Main(string[] args)
        {
            List<Order> orders = new List<Order> {
              new Order { ID="P", Amount=100 },
              new Order { ID="Q", Amount=200 },
              new Order { ID="R", Amount=300 },
              new Order { ID="S", Amount=400 },
              new Order { ID="T", Amount=500 },
              new Order { ID="U", Amount=600 },
              new Order { ID="V", Amount=700 },
              new Order { ID="W", Amount=800 },
              new Order { ID="X", Amount=900 },
              new Order { ID="Y", Amount=1000 },
              new Order { ID="Z", Amount=1100 }
            };
            List<Customer> customers = new List<Customer> {
              new Customer { ID="A", City="New York", Country="USA", Region="North America", Sales=9999},
              new Customer { ID="B", City="Mumbai", Country="India", Region="Asia", Sales=8888},
              new Customer { ID="C", City="Karachi", Country="Pakistan", Region="Asia", Sales=7777},
              new Customer { ID="D", City="Delhi", Country="India", Region="Asia", Sales=6666},
              new Customer { ID="E", City="São Paulo", Country="Brazil", Region="South America", Sales=5555 },
              new Customer { ID="F", City="Moscow", Country="Russia", Region="Europe", Sales=4444 },
              new Customer { ID="G", City="Seoul", Country="Korea", Region="Asia", Sales=3333 },
              new Customer { ID="H", City="Istanbul", Country="Turkey", Region="Asia", Sales=2222 },
              new Customer { ID="I", City="Shanghai", Country="China", Region="Asia", Sales=1111 },
              new Customer { ID="J", City="Lagos", Country="Nigeria", Region="Africa", Sales=1000 },
              new Customer { ID="K", City="Mexico City", Country="Mexico", Region="North America", Sales=2000 },
              new Customer { ID="L", City="Jakarta", Country="Indonesia", Region="Asia", Sales=3000 },
              new Customer { ID="M", City="Tokyo", Country="Japan", Region="Asia", Sales=4000 },
              new Customer { ID="N", City="Los Angeles", Country="USA", Region="North America", Sales=5000 },
              new Customer { ID="O", City="Cairo", Country="Egypt", Region="Africa", Sales=6000 },
              new Customer { ID="P", City="Tehran", Country="Iran", Region="Asia", Sales=7000 },
              new Customer { ID="Q", City="London", Country="UK", Region="Europe", Sales=8000 },
              new Customer { ID="R", City="Beijing", Country="China", Region="Asia", Sales=9000 },
              new Customer { ID="S", City="Bogotá", Country="Colombia", Region="South America", Sales=1001 },
              new Customer { ID="T", City="Lima", Country="Peru", Region="South America", Sales=2002 }
           };

            var customerIDs =
               from c in customers
               select c.ID
               ;
            var orderIDs =
               from o in orders
               select o.ID
               ;
            var customersWithOrders = customerIDs.Intersect(orderIDs);
            Console.WriteLine("Customers with Orders:");
            foreach (var item in customersWithOrders)
            {
                Console.Write("{0} ", item);
            }
            Console.WriteLine();

            Console.WriteLine("Orders with no customers:");
            var ordersNoCustomers = orderIDs.Except(customerIDs);
            foreach (var item in ordersNoCustomers)
            {
                Console.Write("{0} ", item);
            }
            Console.WriteLine();

            Console.WriteLine("All Customer and Order IDs:");
            var allCustomerOrderIDs = orderIDs.Union(customerIDs);
            foreach (var item in allCustomerOrderIDs)
            {
                Console.Write("{0} ", item);
            } 
            Console.WriteLine();

            Console.Write("Program finished, press Enter/Return to continue:");
            Console.ReadLine();
        }
    }

}

Input result:

Small note:

Intersect
Description: Get the intersection item; delay. That is, getting the same items (intersection) from different collections. That is, first traverse the first set to find all unique elements, then traverse the second set, compare each element with the previously found element, and return all elements that appear in both sets.
Except (AND NOT)
Description: Exclude intersecting items; delay. That is, delete items from one collection that are the same as those from another collection. First traverse the first set to find all unique elements, then traverse the second set and return all elements in the second set that do not appear in the previously obtained set of elements.
Union (merge)
Description: Connect different collections and automatically filter the same items; delay. That is, the two collections are merged and identical items are filtered.

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