LINQ to SQL语句 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();        }    }}

输入结果：

小注：

Intersect（相交）
        说明：取相交项；延迟。即是获取不同集合的相同项（交集）。即先遍历第一个集合，找出所有唯一的元素，然后遍历第二个集合，并将每个元素与前面找出的元素作对比，返回所有在两个集合内都出现的元素。
Except（与非）
        说明：排除相交项；延迟。即是从某集合中删除与另一个集合中相同的项。先遍历第一个集合，找出所有唯一的元素，然后再遍历第二个集合，返回第二个集合中所有未出现在前面所得元素集合中的元素。
Union（合并）
        说明：连接不同的集合，自动过滤相同项；延迟。即是将两个集合进行合并操作，过滤相同的项。