LINQ to Objects : How to Join with Data in Another Sequence (part 6) - One-to-Many Joins - The join Operator

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3. One-to-Many Joins

A one-to-many join allows a subgroup collection of related elements to a parent element in another sequence. The most common example of a one-to-many join is a listing of Customers, each with a collection of Orders. The same caveat applies to one-to-many joins as warned in one-to-one joins. LINQ to Objects will not perform joins as well as a relational database system like SQL Server. Joining data is what DBMSs are good at, and although you can perform these joins in LINQ to Objects, it should be reserved for small data volumes. Do the heavy lifting in SQL Server and fine-tuning in LINQ to Objects.

There are a number of ways to achieve one-to-many joins using LINQ, and these include

  • Using the join/into combination (or GroupJoin) operator

  • Using a subquery in the select projection

  • Using the ToLookup operator

Each of these approaches are covered in detail next, including their performance behavior, but Table 4 summarizes the different performance traits for each syntax when working with many records at one time or only joining to a single record.

Table 4. One-to-Many Join Techniques and Their Performance Comparisons
TechniqueMany outer record performanceSingle outer record performanceComments
Using the join/into combination (or GroupJoin) operatorGenerally poor; better as inner sequence size growsWorstWhen run over one million iterations, this performed slower than both other options. Unless trying to stay syntactically similar to SQL, use either a subquery or lookup technique. This technique is more effective than the subquery technique as the inner sequence size grows. Break even at around five records. 
Using a subquery in the select projectionGood for small inner sequences only or when ordering of the inner sequence is importantBestThis technique was only marginally slower than the lookup technique. The lookup technique traverses the inner sequence only once; this technique will traverse the inner sequence multiple times. Use this technique when you need to order the inner sequence elements.
Using the ToLookup operatorBestGoodIf performance is a key requirement, then use this technique.

The identical sample data used in the previous section on one-to-one joins (Table 2) will continue to be used throughout this section unless otherwise specified.

The join/into (GroupJoin) Operator

A small variation of the query expression join syntax allows joined records to be captured in a subgroup

Listing 17 demonstrates how to achieve a one-to-many group relationship using the join and into keyword combination. The Console output is shown in Output 7. The into keyword captures each group, and this group can be accessed in the select projection and subsequently accessed as a property of each result element.

Listing 7. One-to-Many Join using the join/into keyword combination—see Output 7
var customers = Customer.SampleData();
var orders = Order.SampleData();

var q1 = from c in customers
join o in orders on
c.CustomerID equals o.CustomerID
into cust_orders
select new
LastName = c.LastName,
Orders = cust_orders

foreach (var customer in q1)
Console.WriteLine("Last name: {0}", customer.LastName);
foreach (var order in customer.Orders)
Console.WriteLine(" - Order number: {0}", order.OrderNumber);

Output 7.
Last name: Gottshall
- Order number: Order 1
- Order number: Order 2
Last name: Valdes
Last name: Gauwain
Last name: Deane
- Order number: Order 3
- Order number: Order 4
Last name: Zeeman
- Order number: Order 5
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