C# in Depth

Joins30111.5.2 Group joins with join … into clausesWe’ve seen that the result sequence from a normal join clause consistsof pairs of elements, one from each of the input sequences. A group joinlooks similar in terms of the query expression but has a significantly differentoutcome. Each element of a group join result consists of an elementfrom the left sequence (using its original range variable), and alsoa sequence of all the matching elements of the right sequence, exposed asa new range variable specified by the identifier coming after into in thejoin clause.Let’s change our previous example to use a group join. Listing 11.13 again shows allthe defects and the notifications required for each one, but breaks them out in a perdefectmanner. Pay particular attention to how we’re displaying the results, and to thenested foreach loop.ResultcontainsembeddedsubsequencesListing 11.13Joining defects and subscriptions with a group joinvar query = from defect in SampleData.AllDefectsjoin subscription in SampleData.AllSubscriptionson defect.Project equals subscription.Projectinto groupedSubscriptionsselect new { Defect=defect,Subscriptions=groupedSubscriptions };foreach (var entry in query){Console.WriteLine(entry.Defect.Summary);foreach (var subscription in entry.Subscriptions){Console.WriteLine (" {0}", subscription.EmailAddress);}}The Subscriptions property of each entry is the embedded sequence of subscriptionsmatching that entry’s defect. Figure 11.7 shows how the two initial sequences arecombined.One important difference between an inner join and a group join—and indeedbetween a group join and normal grouping—is that for a group join there’s a one-toonecorrespondence between the left sequence and the result sequence, even if someof the elements in the left sequence don’t match any elements of the right sequence.This can be very important, and is sometimes used to simulate a left outer join fromSQL. The embedded sequence is empty when the left element doesn’t match any rightelements. As with an inner join, a group join buffers the right sequence but streamsthe left one.Listing 11.14 shows an example of this, counting the number of bugs created oneach day in August. It uses a DateTimeRange (as described in chapter 6) as the leftsequence, and a projection that calls Count on the embedded sequence in the resultof the group join.Licensed to Rhona Hadida