Linq子查询

 

             NorthwindEntities db = new NorthwindEntities();
            //查找包含a并转换为大写
            List<string> query = (from q in db.Customers select q.CompanyName).ToList();
            //查询长度最短的
            //利用类库中函数
            var query1 = from q1 in query where q1.Length == query.Min(c => c.Length) select q1;
            //注意：q1的作用范围已进入到子查询，所以子查询应重新声明变量
            var query2 = from q1 in query where q1.Length == (from q2 in query orderby q2.Length select q2.Length).First() select q1;
            var query3 = query.Where(q1 => q1.Length == query.OrderBy(q2 => q2.Length).Select(q2 => q2.Length).First());
                       
            //into字句:用于保存临时查询结果.注意，into后面已无法在接from字句了;
            //之前的查询变量经过q1之后，则不可见.这个道理就和lambda中的变量一样，
            //出了作用域，就无法使用了
            List<Customers> queryInto = (from q in db.Customers
                                   where q.CustomerID.Contains("aa")
                                   select q
                                       into q1
                                       where q1.CustomerID.Length > 4
                                       select q1).ToList();

            //let字句:将一个查询字句作为临时表量
            var queryLet = from q in query let xx = from qq in query where qq == "YOYOYO" select qq select q+"_"+xx.First();
            foreach (var str in queryLet)
            {
                Console.WriteLine(str);
            }

            //int[] numbers = new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };   //传统下的子查询做法 
            //var query55 = from num in numbers  select num * (from n in numbers  where n % 2 == 0  select n).Count();
            //var query66 = from num in numbers let evenNumbers = from n in numbers where n % 2 == 0 select n select num * evenNumbers.Count(); 
            //foreach (var item in query66) { Console.WriteLine(item); } 
            Console.ReadKey();