LinQ查询基础（一）LinQ查询表达式

1.1 用from子句指定数据源

from localVar in dataSource

一般情况下，编译器会根据数据源类型自动为from子句的localVar元素制定数据类型

当然也可自己指定数据类型，编译器不会检查数据类型，也就是当数据类型错误的时候，编译器不会报错，只有在查询结果被使用的时候，才会进行检查从而报错。

            int[] ary = { 1, 2, 3, 5, 7, 9 };
            var query =
                from v in ary
                select v;

输出结果为：

1.2 用select子句指定目标数据

            Student[] arry =
            {
                new Student("张三",20,"男"),
            new Student("李四", 22, "男"),
            new Student("王五", 21, "女"),
            new Student("赵六", 21, "女")        };

            var query =
                from v in arry
                select new { v.Name,v.Age,v.Gender};

            foreach (var v in query)
                Console.WriteLine("{0} ", v);

输出结果：

1.3 用where子句指定筛选条件

            Student[] arry =
            {
                new Student("张三",20,"男"),
            new Student("李四", 22, "男"),
            new Student("王五", 21, "女"),
            new Student("赵六", 21, "女")        };

            var query =
                from v in arry
                where v.Age > 20
                where v.Gender == "男"
                select new { v.Name, v.Age, v.Gender };

            foreach (var v in query)
                Console.WriteLine("{0} ", v);

可以使用多个并列的where子句来进行条件过滤

输出结果：

1.4 用orderby 子句排序(ascending 升序  descending 降序)

Student[] arry =
            {
                new Student("张三",20,"男"),
            new Student("李四", 22, "男"),
            new Student("王五", 21, "女"),
            new Student("赵六", 21, "女")        };

            var query =
                from v in arry
                orderby v.Name.Length ascending, v.Age descending 
                select new { v.Name, v.Age, v.Gender };

            foreach (var v in query)
                Console.WriteLine("{0} ", v);

当一个LinQ查询中出现多个orderby的时候，只有最后一句有效

输出结果：

1.5用group子句进行分组

Student[] arry =
            {
                new Student("张三",20,"男"),
            new Student("李四", 22, "男"),
            new Student("王五", 21, "女"),
            new Student("赵六", 21, "女")        };

            var query =
                from v in arry
                group v by v.Gender;

            foreach (var v in query)
            {
                Console.WriteLine("{0} ", v.Key);
                foreach (var x in v)
                {
                    Console.WriteLine("\t{0}-{1}-{2}", x.Name, x.Age, x.Gender);
                }
            }

输出结果：

1.6 用from子句进行复合查询

    class Program
    {

        class Lessn
        {
            public string Name { get; set; }
            public int Score { get; set; }
            public Lessn(string name, int s)
            {
                Name = name;
                Score = s;
            }
        }

        class Student
        {
            public string Name { get; set; }
            public int Age { get; set; }
            public string Gender { get; set; }
            public List<Lessn> Scores = new List<Lessn>();
            public Student(string name, int age, string gender, params Lessn[] lessn)
            {
                Name = name;
                Age = age;
                Gender = gender;
                foreach (var x in lessn)
                    Scores.Add(x);
            }
        }

        static void Main(string[] args)
        {
            Student[] arry =
            {
                new Student("张三",20,"男",new Lessn("数学",81),new Lessn("英语",70),new Lessn("语文",60)),
            new Student("李四", 22, "男",new Lessn("数学",90),new Lessn("英语",80),new Lessn("语文",50)),
            new Student("王五", 21, "女",new Lessn("数学",81),new Lessn("英语",55),new Lessn("语文",50)),
            new Student("赵六", 21, "女",new Lessn("数学",50),new Lessn("英语",70),new Lessn("语文",40))        };

            var query =
                from v in arry
                from v2 in v.Scores
                group new { v.Name, v2 } by v.Name;

            foreach (var v in query)
            {
                Console.WriteLine("{0} ", v.Key);
                foreach (var x in v)
                {
                    Console.WriteLine("\tName = {0},  scr:{1} == {2}分", x.Name, x.v2.Name, x.v2.Score);
                }
            }

            Console.ReadKey();
        }
    }

输出结果：

1.7用join子句进行联接

内部联接

join element in dataSource on exp1 equals exp2

分组联接

join element in dataSource on exp1 equals exp2 into graName

左外部连接比较复杂 就实例说明

            int[] arry1 = { 5, 15, 23 };
            int[] arry2 = { 10, 20, 30, 50, 70, 90 };
            //通过对分组联接的结果调用DefaultIfEmpty()来执行左外部联接
            //DefaultIfEmpty()方法从列表中获取指定元素，如果列表为空，返回默认值。
            var query =
                from v in arry1
                join v2 in arry2 on v % 5 equals v2 % 5 into v3
                from v4 in v3.DefaultIfEmpty()
                select new { V = v, V2 = v4 };

            foreach(var v in query)
            {
                Console.WriteLine("{0}", v);
            }

输出结果: