29.LINQ初探

本页内容：

1.LINQ查询语言的组成部分；

2.使用LINQ方法语法；

3.排序查询结果 orderby 字句；

4.聚合运算符；

5.查询复杂对象；

6.投影：在查询中创建新对象；

7.Distinct()、Any()、All()、First()、FirtOrDefault()、Take()、Skip()运算符；

8.组合查询

9.集运算符

10.联合 Join运算符

1.LINQ查询语言的组成部分

static void Main(string[] args)
        {
            string[] names = { "Alonso", "Zheng", "Smith", "Jones", "Smythe", "Small", "Ruiz", "Hsieh", "Jorgenson", "Ilyich", "Singh", "Samba", "Fatimah" };

            var queryResults=from n in names
                             where n.StartsWith("S")
                             select n;
            Console.WriteLine("Names beginning with S:");
            foreach (var item in queryResults)
            {
                Console.WriteLine(item);
            }

            Console.ReadLine();
        }

该例子使用了LINQ查询语法声明了一个LINQ查询

            var queryResults=from n in names
                             where n.StartsWith("S")
                             select n;

该语句包括以下几个组成部分：

1.1 声明查询结果变量 var queryResults=；

1.2指定数据源 from n in names ；

1.3指定条件 where n.StartsWith("S")；

1.4指定查询元素 select n;

注：LINQ是延迟执行的，它是在实际使用时执行的，不是在声明的时候执行的，查询结果变量queryResults只是保存了一个查询计划。本例中是在foreach循环时执行LINQ查询的。

2.使用LINQ方法语法

使用LINQ查询有多种方式，可以使用LINQ查询语法，如上例，也可以使用LINQ方法语法，也称显式语法。查询语法是LINQ查询的首选方式，因为查询语法比较通俗易懂，但是也需要用到方法语法，因为一些LINQ功能不能通过查询语法来查询，如聚合运算符。

        static void Main(string[] args)
        {
            string[] names = { "Alonso", "Zheng", "Smith", "Jones", "Smythe", "Small", "Ruiz", "Hsieh", "Jorgenson", "Ilyich", "Singh", "Samba", "Fatimah" };

            var queryResults=names.Where(n=> n.StartsWith("S"));

            Console.WriteLine("Names beginning with S:");
            foreach (var item in queryResults)
            {
                Console.WriteLine(item);
            }

            Console.ReadLine();
        }

使用LINQ方法语法只需要调用对应的方法即可，这个方法需要传送一个匿名方法作为参数，一般使用lamabda表达式创建匿名方法。

3.排序查询结果 orderby 字句

可以使用orderby 子句对查询结果进行排序，如下对结果按字母对名字进行升序排序

        static void Main(string[] args)
        {
            string[] names = { "Alonso", "Zheng", "Smith", "Jones", "Smythe", "Small", "Ruiz", "Hsieh", "Jorgenson", "Ilyich", "Singh", "Samba", "Fatimah" };

            var queryResults=from n in names
                             where n.StartsWith("S")
                             orderby n
                             select n;
            Console.WriteLine("Names beginning with S:");
            foreach (var item in queryResults)
            {
                Console.WriteLine(item);
            }

            Console.ReadLine();
        }

orderby子句可以按照任意表达式对结果进行排序，如按照按照名字最后一个字母排序 orderby n.Substring(n.Length-1)

orderby默认升序排序，如果要降序排序可以添加descending关键字 orderby n descending

用方法语法排序

        static void Main(string[] args)
        {
            string[] names = { "Alonso", "Zheng", "Smith", "Jones", "Smythe", "Small", "Ruiz", "Hsieh", "Jorgenson", "Ilyich", "Singh", "Samba", "Fatimah" };

            var queryResults = names.Where(n => n.StartsWith("s")).OrderBy(n => n);
            Console.WriteLine("Names beginning with S:");
            foreach (var item in queryResults)
            {
                Console.WriteLine(item);
            }

            Console.ReadLine();
        }

给元素按降序排序可以使用OrderByDescending

var queryResults = names.Where(n => n.StartsWith("s")).OrderByDescending(n => n.Substring(n.Length-1));

多级排序

有时候需用用到多个字段来进行排序，比如按照销量降序，国家升序，城市降序，就需要联合使用OrderBy\OrderByDescending和ThenBy/ThenByDescending

var queryResult = customers .OrderByDescending(n=>n.Sales).ThenBy(n=>n.Country).ThenByDescending(n=>n.City);

4.聚合运算符

        static void Main(string[] args)
        {
            int[] numbers = GenerateLotsOfNumbers();

            var queryResults = from n in numbers
                               where n >
                               select n;
            Console.WriteLine("Count of numbers >1000:"+queryResults.Count());
            Console.WriteLine("Max of numbers >1000:" + queryResults.Max());
            Console.WriteLine("Min of numbers >1000:" + queryResults.Min());
            Console.WriteLine("Average of numbers >1000:" + queryResults.Average());
            Console.WriteLine("Sum of numbers >1000:" + queryResults.Sum(n=>(long)n));//必须转换为long，因为sum太大超过了int存储的范围
            Console.ReadLine();
        }

使用聚合运算符在不进行循环的情况下分析结果，特别是对数字类型的结果特别有用。

5.查询复杂对象

上面的例子中查询的对象都是数字，字符串等简单类型，也可以使用LINQ查询对象等复杂数据类型

    public 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; }

        public override string ToString()
        {
            return "ID:" + ID + ",City:" + City + ",Country:" + Country + ",Region:" + Region + ",Sales:"+Sales;
        }
    }

        static void Main(string[] args)
        {
            List<Customer> customers = new List<Customer> {
                new Customer {ID="A" ,City ="New York" ,Country ="USA", Region="North America", Sales =},
                new Customer {ID="B" ,City ="Munbai" ,Country ="India", Region="Asia", Sales =},
                new Customer {ID="C" ,City ="Karachi" ,Country ="Pakistan", Region="Asia", Sales =},
                new Customer {ID="D" ,City ="Delhi" ,Country ="India", Region="Asia", Sales =},
                new Customer {ID="E" ,City ="S o Paulo" ,Country ="Brazil", Region="South America", Sales =},
            };

            var queryResults = from n in customers
                               where n.Region == "Asia"
                               select n;
            Console.WriteLine("Customers in Asin:");
            foreach (var item in queryResults)
                Console.WriteLine(item);
            Console.ReadLine();
        }

6.投影：在查询中创建新对象

投影（Projection）是在LINQ查询中从其他数据类型创建新数据类型的技术术语，select关键字是投影运算符

    public 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; }

        //public override string ToString()
        //{
        //    return "ID:" + ID + ",City:" + City + ",Country:" + Country + ",Region:" + Region + ",Sales:"+Sales;
        //}
    }

        static void Main(string[] args)
        {
            List<Customer> customers = new List<Customer> {
                new Customer {ID="A" ,City ="New York" ,Country ="USA", Region="North America", Sales =},
                new Customer {ID="B" ,City ="Munbai" ,Country ="India", Region="Asia", Sales =},
                new Customer {ID="C" ,City ="Karachi" ,Country ="Pakistan", Region="Asia", Sales =},
                new Customer {ID="D" ,City ="Delhi" ,Country ="India", Region="Asia", Sales =},
                new Customer {ID="E" ,City ="S o Paulo" ,Country ="Brazil", Region="South America", Sales =},
            };

            var queryResults = from n in customers
                               where n.Region == "Asia"
                               select new { n.Region , n.Country ,n.City ,n.Sales };
            Console.WriteLine("Customers in Asin:");
            foreach (var item in queryResults)
                Console.WriteLine(item);
            Console.ReadLine();
        }

投影创建的匿名类型不用提供ToString重新方法，因为编译器提供了默认的重新方法，以名值对的方式输出对象（正常类如果不重写Tostring方法输出的是类名，如ConsoleApplication1.Customer）

7.Distinct()、Any()、All()、First()、FirtOrDefault()、Take()、Skip()运算符

Distinct()和SQL distinct一样返回数据集中的唯一值

            var queryResult = customers.Select(n=>n.Region).Distinct();
            var queryResult2 = customers.Select(n =>new {n.Country,n.City }).Distinct();

Any()检查数据集是否有任何一个元素满足条件，如果有返回true，没有返回false；All()检查数据集是否所有元素都符合条件

            bool anyUSA = customers.Any(n=>n.Country=="UAS");
            var allAsia = customers.All(n=>n.Region =="Asia");

First()返回数据集第一个满足条件的元素，如果没有符合条件的元素将抛出异常；FirstOrDefault()返回满足条件的第一个元素，与First不同的是如果没有符合条件的元素返回空，不会抛出异常

            //查询语法
            var queryResult=(from n in customers select n).First(n => n.Country == "USA");//查询语法和方法语法混用

            //方法语法
            var customerUSA = customers.First(n => n.Country == "USA");
            var customerAsia = customers.FirstOrDefault(n => n.Region == "Asia");

take()取结果集前N条，skip()略过前N条取剩余的

            var salesTop3 = customers.OrderByDescending(n=>n.Sales).Take();
            var salesLeft = customers.OrderByDescending(n => n.Sales).Skip();

 8.组合查询

组合查询类似SQL group by，允许把数据分组，按组来排序，计算聚合值和进行比较。

            var queryResult = from n in customers
                              group n by n.Region into cg
                              select new { TotalSales = cg.Sum(c => c.Sales), Region = cg.Key };

            var orderResult = from cg in queryResult
                              orderby cg.TotalSales descending
                              select cg;

9.集运算符

使用集运算符，可以快速的对两个结果集进行操作。集运算符要求集成员有相同的类型，才能得到希望的结果

    public 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; }

        public override string ToString()
        {
            return "ID:" + ID + ",City:" + City + ",Country:" + Country + ",Region:" + Region + ",Sales:" + Sales;
        }
    }

    public class Order
    {
        public string ID { get; set; }
        public decimal Amount { get; set; }
    }

            List<Customer> customers = new List<Customer> {
                new Customer {ID="A" ,City ="New York" ,Country ="USA", Region="North America", Sales =},
                new Customer {ID="B" ,City ="Munbai" ,Country ="India", Region="Asia", Sales =},
                new Customer {ID="C" ,City ="Karachi" ,Country ="Pakistan", Region="Asia", Sales =},
                new Customer {ID="D" ,City ="Delhi" ,Country ="India", Region="Asia", Sales =},
                new Customer {ID="E" ,City ="S o Paulo" ,Country ="Brazil", Region="South America", Sales =},
            };

            List<Order> orders = new List<Order>
            {
                new Order {ID ="A",Amount=},
                new Order {ID ="B",Amount=},
                new Order {ID ="F",Amount=},
            };

            var customerIds = from n in customers select n.ID;
            var orderIds = from n in orders select n.ID;

            //有订单的客户ID
            var customerWithOrders= customerIds .Intersect(orderIds);

            //没有客户的订单
            var ordersNoCustomers = orderIds.Except(customerIds);

            //客户和订单的ID集合
            var allCustomersOrdersIds = customerIds.Union(orderIds);

10.联合 Join运算符

Join运算符类似SQL Join操作，可以把两个数据集用关键字段连接起来。

            var queryResult = from n in customers
                              join o in orders on n.ID equals o.ID
                              select new { n.ID ,n.City ,SaleBefore=n.Sales ,OrderAmount=o.Amount ,SaleAfter=n.Sales +o.Amount };