LINQ学习（四）：From子句

说明：查询表达式必须以 from 子句开头。另外，查询表达式还可以包含子查询，子查询也是以 from 子句开头。SQL命令中from指的是数据表，LINQ中from 子句中引用的数据源的类型必须为 IEnumerable、IEnumerable<T> 或一种派生类型（如 IQueryable<T>）。

1.复合from子句

说明：如果一个数据源里面又包含了一个或多个集合列表，那么应该使用复合的from子句来进行查询。

查询分数小于60分学生的姓名和分数：

        class Student{public string Name { get; set; }public List<int> Scores { get; set; }}static void Main(string[] args){List<Student> students = new List<Student>{new Student {Name="Terry", Scores=new List<int> {97, 72, 81, 60}},new Student {Name="AI", Scores=new List<int> {75, 84, 91, 39}},new Student {Name="Wade", Scores=new List<int> {88, 94, 65, 85}},new Student {Name="Tracy", Scores=new List<int>{97, 89, 85, 82}},new Student {Name="Kobe", Scores=new List<int> {35, 72, 91, 70}} };var query = from student in studentsfrom score in student.Scoreswhere score < 60select new { name = student.Name, score };foreach (var student in query){Console.WriteLine("{0},{1}", student.name, student.score);//AI,39//Kobe,35
            }Console.ReadKey();}

2.使用多个 from 子句执行联接

说明：复合 from 子句用于访问单个数据源中的内部集合。不过，查询还可以包含多个可从独立数据源生成补充查询的 from 子句。

交叉联接：

            char[] char1 = { 'A', 'B', 'C' };char[] char2 = { 'a', 'b', 'c' };var query =from c1 in char1from c2 in char2select new { c1, c2 };Console.WriteLine("交叉联接:");foreach (var result in query){Console.WriteLine("{0}", result);//交叉联接://{ c1 = A, c2 = a }//{ c1 = A, c2 = b }//{ c1 = A, c2 = c }//{ c1 = B, c2 = a }//{ c1 = B, c2 = b }//{ c1 = B, c2 = c }//{ c1 = C, c2 = a }//{ c1 = C, c2 = b }//{ c1 = C, c2 = c }}