101 LINQ Samples: Join Operators

最新推荐文章于 2021-03-29 09:36:21 发布

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最新推荐文章于 2021-03-29 09:36:21 发布

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原文链接：http://www.cnblogs.com/zjz008/archive/2011/03/13/1982980.html

本文深入探讨了使用LINQ进行高效序列连接的方法，包括交叉连接、分组连接、左外连接等高级技巧，通过实例展示了如何基于两个序列的键表达式进行元素匹配。

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Cross Join

This sample shows how to efficiently join elements of two sequences based on equality between key expressions over the two.

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
GuaranÃ¡ FantÃ¡stica: Beverages
Sasquatch Ale: Beverages
Steeleye Stout: Beverages
CÃ´te de Blaye: Beverages
Chartreuse verte: Beverages
Ipoh Coffee: Beverages
Laughing Lumberjack Lager: Beverages
Outback Lager: Beverages
RhÃ¶nbrÃ¤u Klosterbier: Beverages
LakkalikÃ¶Ã¶ri: Beverages
Aniseed Syrup: Condiments
Chef Anton's Cajun Seasoning: Condiments
Chef Anton's Gumbo Mix: Condiments
Grandma's Boysenberry Spread: Condiments
Northwoods Cranberry Sauce: Condiments
Genen Shouyu: Condiments
Gula Malacca: Condiments
Sirop d'Ã©rable: Condiments
Vegie-spread: Condiments
Louisiana Fiery Hot Pepper Sauce: Condiments
Louisiana Hot Spiced Okra: Condiments
Original Frankfurter grÃ¼ne SoÃŸe: Condiments
Queso Cabrales: Dairy Products
Queso Manchego La Pastora: Dairy Products
Gorgonzola Telino: Dairy Products
Mascarpone Fabioli: Dairy Products
Geitost: Dairy Products
Raclette Courdavault: Dairy Products
Camembert Pierrot: Dairy Products
Gudbrandsdalsost: Dairy Products
Flotemysost: Dairy Products
Mozzarella di Giovanni: Dairy Products
Ikura: Seafood
Konbu: Seafood
Carnarvon Tigers: Seafood
Nord-Ost Matjeshering: Seafood
Inlagd Sill: Seafood
Gravad lax: Seafood
Boston Crab Meat: Seafood
Jack's New England Clam Chowder: Seafood
Rogede sild: Seafood
Spegesild: Seafood
Escargots de Bourgogne: Seafood
RÃ¶d Kaviar: Seafood

Group Join

Using a group join you can get all the products that match a given category bundled as a sequence.

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

Result

Beverages:
Chai
Chang
GuaranÃ¡ FantÃ¡stica
Sasquatch Ale
Steeleye Stout
CÃ´te de Blaye
Chartreuse verte
Ipoh Coffee
Laughing Lumberjack Lager
Outback Lager
RhÃ¶nbrÃ¤u Klosterbier
LakkalikÃ¶Ã¶ri
Condiments:
Aniseed Syrup
Chef Anton's Cajun Seasoning
Chef Anton's Gumbo Mix
Grandma's Boysenberry Spread
Northwoods Cranberry Sauce
Genen Shouyu
Gula Malacca
Sirop d'Ã©rable
Vegie-spread
Louisiana Fiery Hot Pepper Sauce
Louisiana Hot Spiced Okra
Original Frankfurter grÃ¼ne SoÃŸe
Vegetables:
Dairy Products:
Queso Cabrales
Queso Manchego La Pastora
Gorgonzola Telino
Mascarpone Fabioli
Geitost
Raclette Courdavault
Camembert Pierrot
Gudbrandsdalsost
Flotemysost
Mozzarella di Giovanni
Seafood:
Ikura
Konbu
Carnarvon Tigers
Nord-Ost Matjeshering
Inlagd Sill
Gravad lax
Boston Crab Meat
Jack's New England Clam Chowder
Rogede sild
Spegesild
Escargots de Bourgogne
RÃ¶d Kaviar

Cross Join with Group Join

The group join operator is more general than join, as this slightly more verbose version of the cross join sample shows.

public void Linq104() {     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         select new { Category = c, p.ProductName };       foreach (var v in q)     {         Console.WriteLine(v.ProductName + ": " + v.Category);     } }

Result

Left Outer Join

A so-called outer join can be expressed with a group join. A left outer joinis like a cross join, except that all the left hand side elements get included at least once, even if they don't match any right hand side elements. Note how Vegetablesshows up in the output even though it has no matching products.

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
Chang: Beverages
GuaranÃ¡ FantÃ¡stica: Beverages
Sasquatch Ale: Beverages
Steeleye Stout: Beverages
CÃ´te de Blaye: Beverages
Chartreuse verte: Beverages
Ipoh Coffee: Beverages
Laughing Lumberjack Lager: Beverages
Outback Lager: Beverages
RhÃ¶nbrÃ¤u Klosterbier: Beverages
LakkalikÃ¶Ã¶ri: Beverages
Aniseed Syrup: Condiments
Chef Anton's Cajun Seasoning: Condiments
Chef Anton's Gumbo Mix: Condiments
Grandma's Boysenberry Spread: Condiments
Northwoods Cranberry Sauce: Condiments
Genen Shouyu: Condiments
Gula Malacca: Condiments
Sirop d'Ã©rable: Condiments
Vegie-spread: Condiments
Louisiana Fiery Hot Pepper Sauce: Condiments
Louisiana Hot Spiced Okra: Condiments
Original Frankfurter grÃ¼ne SoÃŸe: Condiments
(No products): Vegetables
Queso Cabrales: Dairy Products
Queso Manchego La Pastora: Dairy Products
Gorgonzola Telino: Dairy Products
Mascarpone Fabioli: Dairy Products
Geitost: Dairy Products
Raclette Courdavault: Dairy Products
Camembert Pierrot: Dairy Products
Gudbrandsdalsost: Dairy Products
Flotemysost: Dairy Products
Mozzarella di Giovanni: Dairy Products
Ikura: Seafood
Konbu: Seafood
Carnarvon Tigers: Seafood
Nord-Ost Matjeshering: Seafood
Inlagd Sill: Seafood
Gravad lax: Seafood
Boston Crab Meat: Seafood
Jack's New England Clam Chowder: Seafood
Rogede sild: Seafood
Spegesild: Seafood
Escargots de Bourgogne: Seafood
RÃ¶d Kaviar: Seafood