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Linq Joins with SelectMany

, 19 Jan 2011 CPOL
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Linq Joins with SelectMany
SelectMany projects each element of a sequence to an IEnumerable and flattens the resulting sequences into one sequence. In this post, I am going to show how you can use SelectMany extension method to achieve the join between related tables easily without writing long queries.
 
To understand this, consider the example below:

As shown in the above image, customer is having relation with the order table.
 
Scenario
Find all of the users who have an order.
 
To achieve the above requirement, you need to apply an inner join between the tables. The Linq query to do this is:
var CustomerWithOrders = from c in Customers
  from o in c.Orders
    select new { 
           c.Name, 
           p.Description, 
           o.Price 
          }
The query above inner joins the customer table with the order table and returns those customers having orders.
 
The shorter way to achieve the above is to use SelectMany:
Customers.SelectMany (
      c => c.Orders, 
      (c, o) => 
         new  
         {
            Name = c.Name, 
            Description = o.Description, 
            Price = o.Price
         }
   ) 
 
Scenario 2
Find all of the users who have an order and display N/A for the users who do not.
 
To achieve the above requirement, you need to apply an Outer join between the tables. The Linq query to do this is:
var CustomerWithOrders = from c in Customers
 from o in c.Orders.DefaultIfEmpty()
 select new { 
         Name =c.Name,
         Description = ( o.Description ?? "N/A"), 
         Price = (((decimal?) o.Price) ?? 0)  
        }
 
The query above outer joins the customer table with the order table and returns all of the customers with or without orders.
 
But with the SelectMany, the query is:
Customers.SelectMany (
      c => c.Orders.DefaultIfEmpty (), 
      (c, o) => 
         new  
         {
            Name = c.Name, 
            Description = (o.Description ?? "N/A"), 
            Price = ((Decimal?)(o.Price) ?? 0)
         }
   ) 
 
Summary
SelectMany is a way to flatten several results into a single sequence.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Pranay Rana
Software Developer (Senior) GMind Solusion
India India

Microsoft C# MVP (12-13)

 
Hey, I am Pranay Rana, working as a ITA in MNC. Web development in Asp.Net with C# and MS sql server are the experience tools that I have had for the past 5.5 years now.
 
For me def. of programming is : Programming is something that you do once and that get used by multiple for many years
 

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Comments and Discussions

 
GeneralReason for my vote of 5 very nice Pinmemberdpflovely24-Jan-11 22:22 
GeneralRe: thanks you can also vote my articles if you like it which is... PinmemberPranay Rana24-Jan-11 23:16 
GeneralI edited the above for grammar/readability and redid the sum... PinmemberAndrew Rissing18-Jan-11 5:09 

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