LINQ to SQL语句 Union/Intersect/Except

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。

Linq to SQL中的ColumnAttribute中的Expression 在Linq to SQL中，使用ColumnAttribute特性来关联数据库和实体类。这个Atrribute也有很多属性可以设置。其中让人比较迷糊的是Expression，也折磨了我好几天才弄明白（因为官方的例子给的也让人迷糊，基本运行都无法通过）。
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