Linq select 语法

文档：https://code.msdn.microsoft.com/101-LINQ-Samples-3fb9811b

1.可以对查询出来的结果做一些转换，下面的例子在数组中查找以"B"开头的名字，然后全部转成小写输出

string[] names = { "Jack", "Bob", "Bill", "Catty", "Willam" };
            var rs = from n in names
                     where n.StartsWith("B")
                     select n.ToLower();
            foreach (var item in rs)
            {
                Response.Write(item+"<br />");
            }

2.返回匿名类型，只返回需要的信息

var users = from c in UserList
                        where c.Mobile.StartsWith("135")
                        select new
                               {
                                   Name = c.Name,
                                   Contact = c.Address + " " + c.Mobile
                               };

3.使用原数组，产生两组新数组

public void Linq9()
{
    string[] words = { "aPPLE", "BlUeBeRrY", "cHeRry" }; 

    var upperLowerWords =
        from w in words
        select new { Upper = w.ToUpper(), Lower = w.ToLower() }; 

    foreach (var ul in upperLowerWords)
    {
        Console.WriteLine("Uppercase: {0}, Lowercase: {1}", ul.Upper, ul.Lower);
    }
}

4.判断数字是否与他的下标一致

int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

            var numsInPlace = numbers.Select((num, index) => new { Num = num, InPlace = (num == index) });

            Console.WriteLine("Number: In-place?");
            foreach (var n in numsInPlace)
            {
                Response.Write(string.Format("{0}: {1}<br />", n.Num, n.InPlace));
            }

5.获得A小于B的所有可能的集合，返回 拿A中的每个元素和B中的每个元素对比的结果

int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 };
    int[] numbersB = { 1, 3, 5, 7, 8 }; 

    var pairs =
        from a in numbersA
        from b in numbersB
        where a < b
        select new { a, b }; 

    Console.WriteLine("Pairs where a < b:");
    foreach (var pair in pairs)
    {
        Console.WriteLine("{0} is less than {1}", pair.a, pair.b);
    }

6.从一个集合的子集中过滤，金额小于500的订单

List<Customer> customers = GetCustomerList();

            var orders =
                from c in customers
                from o in c.Orders
                where o.Total < 500.00
                select new { c.CustomerID, o.OrderID, o.Total };

7.查询所有大于指定日期的用户订单

List<Customer> customers = GetCustomerList(); 
    var orders =
        from c in customers
        from o in c.Orders
        where o.OrderDate >= new DateTime(1998, 1, 1)
        select new { c.CustomerID, o.OrderID, o.OrderDate };

延伸：

List<Customer> customers = GetCustomerList(); 
  
    DateTime cutoffDate = new DateTime(1997, 1, 1); 
  
    var orders = 
        from c in customers 
        where c.Region == "WA" 
        from o in c.Orders 
        where o.OrderDate >= cutoffDate 
        select new { c.CustomerID, o.OrderID }; 

8.代替二重for循环

参考：http://www.cnblogs.com/manupstairs/archive/2012/11/27/2790114.html

http://www.cnblogs.com/lyout/archive/2012/11/28/2792622.html

List<Customer> customers = GetCustomerList(); 

    var customerOrders =
        customers.SelectMany(
            (cust, custIndex) =>
            cust.Orders.Select(o => "Customer #" + (custIndex + 1) + " has an order with OrderID " + o.OrderID));

9.Cross join 交叉连接查询

public void Linq102()
{ 

    string[] categories = new string[]{
        "Beverages",
        "Condiments",
        "Vegetables",
        "Dairy Products",
        "Seafood" };  

    List<Product> products = GetProductList(); 

    var q =
        from c in categories
        join p in products on c equals p.Category
        select new { Category = c, p.ProductName }; 

    foreach (var v in q)
    {
        Console.WriteLine(v.ProductName + ": " + v.Category);
    }
}

Result：
Chai: Beverages
Chang: Beverages
Guaraná Fantástica: Beverages
Sasquatch Ale: Beverages
Steeleye Stout: Beverages
Côte de Blaye: Beverages
Chartreuse verte: Beverages
Ipoh Coffee: Beverages

10. Group join 分组查询；将数据按照不同的类型划分，归入到不同的组中。

public void Linq103()
{
    string[] categories = new string[]{
        "Beverages",
        "Condiments",
        "Vegetables",
        "Dairy Products",
        "Seafood" }; 

    List<Product> products = GetProductList(); 

    var q =
        from c in categories
        join p in products on c equals p.Category into ps
        select new { Category = c, Products = ps }; 

    foreach (var v in q)
    {
        Console.WriteLine("组"+v.Category + ":");
        foreach (var p in v.Products)
        {
            Console.WriteLine("   " + p.ProductName);
        }
    }
}

Result：

组Beverages:
Chai
Chang

组Condiments:
Aniseed Syrup
Chef Anton's Cajun Seasoning
Chef Anton's Gumbo Mix

组Vegetables:
Dairy Products:
Queso Cabrales

11.Left Outer Join 左外连接查询，如果连接表没有数据，这使用空值/默认值填充

public void Linq105()
{
    string[] categories = new string[]{
        "Beverages",
        "Condiments",
        "Vegetables",
        "Dairy Products",
        "Seafood" }; 

    List<Product> products = GetProductList(); 

    var q =
        from c in categories
        join p in products on c equals p.Category into ps
        from p in ps.DefaultIfEmpty()
        select new { Category = c, ProductName = p == null ? "(No products)" : p.ProductName }; 

    foreach (var v in q)
    {
        Console.WriteLine(v.ProductName + ": " + v.Category);
    }
}

Result：
Chai: Beverages
Aniseed Syrup: Condiments
(No products): Vegetables
Queso Cabrales: Dairy Products
Konbu: Seafood