LINQ to Objects : How to Join with Data in Another Sequence (part 4)

2.4 One-to-One Join Using a Subquery

A subquery in a queries Select projection can achieve a one-to-one join. In testing, this option did not perform as well as the alternatives and should only be considered for very small (less than ten-element) sequence sizes. This technique simply looks up related data from a second sequence within the first query’s select projection. Listing 4 demonstrates the subquery technique, and the result is shown in Output 4.

Listing 4. One-to-One Join using Subquery syntax—see Output 4

var customers = Customer.SampleData();
var orders = Order.SampleData();

var q2 = from o in orders
         select new
         {
             OrderNumber = o.OrderNumber,
             LastName = (from c in customers
                         where c.CustomerID == o.CustomerID
                         select c.LastName).SingleOrDefault()
         };

foreach (var order in q2)
    Console.WriteLine(
        "Customer: {0}  Order Number: {1}",
        order.LastName.PadRight(9, ' '),
        order.OrderNumber);

Output 4.

Customer: Gottshall   Order Number: Order 1
Customer: Gottshall   Order Number: Order 2
Customer: Deane       Order Number: Order 3
Customer: Deane       Order Number: Order 4
Customer: Zeeman      Order Number: Order 5

2.5 One-to-One Join Using SingleOrDefault Operator

The SingleOrDefault standard query operator returns the single matching element from another sequence or null if a matching value cannot be found (if more than one matching element is found, an exception is thrown). This is ideal for looking up a matching record in another sequence and making that data accessible to your query—in other words, a one-to-one join. This technique is fast and clean when looking up only a single or a few related records.

Listing 5 demonstrates a one-to-one join using the SingleOrDefault technique, which generates the output shown in Output 5.

Listing 5. One-to-One Join using the `SingleOrDefault` operator—see Output 5

var customers = Customer.SampleData();
var orders = Order.SampleData();

var q3 = from o in orders
         let cust = customers
                    .SingleOrDefault(
                        c => c.CustomerID == o.CustomerID)
         select new
         {
             OrderNumber = o.OrderNumber,
             LastName = cust.LastName
         };

foreach (var order in q3)
    Console.WriteLine(
        "Customer: {0}  Order Number: {1}",
        order.LastName.PadRight(9, ' '),
        order.OrderNumber);

2.6 One-to-One Join Using Cross Join

This join type matches traditional relational database join techniques. However, because LINQ to Objects doesn’t optimize the query (by maintaining indexes, building statistics, rewriting queries, and so on) like most good database server products, one-to-one joins using cross joins should be avoided because performance is poor and the alternatives previously shown are just as easy.

This technique cross joins two sequences to get the Cartesian Product (every element against every element) and then uses a where clause to keep only the matching outer and inner records. This achieves the goal of a one-to-one join, but at great expense in performance due to the number of elements enumerated and never passing the filter criteria. Use this technique sparingly and only if collection sizes are very small.

Listing 6 demonstrates a one-to-one join using the cross join/where technique and generates the code shown in Output 6.

Listing 6. One-to-one join using a cross join and `where` filter—see Output 6

var customers = Customer.SampleData();
var orders = Order.SampleData();

var q4 = from o in orders
         from c in customers
         where c.CustomerID == o.CustomerID
         select new
         {
              o.OrderNumber,
              c.LastName
         };

foreach (var order in q4)
    Console.WriteLine(
        "Customer: {0}  Order Number: {1}",
        order.LastName.PadRight(9, ' '),
        order.OrderNumber);

Output 6.

Customer: Gottshall   Order Number: Order 1
Customer: Gottshall   Order Number: Order 2
Customer: Deane       Order Number: Order 3
Customer: Deane       Order Number: Order 4
Customer: Zeeman      Order Number: Order 5