C# in Depth

Joins29711.5 JoinsEach part of the query has been adjusted appropriately: where the original queryexpression referenced user or length directly, if the reference occurs after the letclause, it’s replaced by z.user or z.length. The choice of z as the name here is arbitrary—it’sall hidden by the compiler.If you read the C# 3 language specification on let clauses, you’ll see that the translationit describes is from one query expression to another. It uses an asterisk (*) torepresent the transparent identifier introduced. The transparent identifier is thenerased as a final step in translation. I won’t use that notation in this chapter, as it’s hardto come to grips with and unnecessary at the level of detail we’re going into. Hopefullywith this background, the specification won’t be quite as impenetrable as it mightbe otherwise, should you need to refer to it.The good news is that we can now take a look at the rest of the translations makingup C# 3’s query expression support. I won’t go into the details of every transparentidentifier introduced, but I’ll mention the situations in which they occur. Let’s look atthe support for joins first.If you’ve ever read anything about SQL, you probably have an idea what a database joinis. It takes two tables and creates a result by matching one set of rows against anotherset of rows. A LINQ join is similar, except it works on sequences. Three types of joinsare available, although not all of them use the join keyword in the query expression.We’ll start with the join that is closest to a SQL inner join.11.5.1 Inner joins using join clausesInner joins involve two sequences. One key selector expression is applied to each elementof the first sequence and another key selector (which may be totally different) isapplied to each element of the second sequence. The result of the join is a sequenceof all the pairs of elements where the key from the first element is the same as the keyfrom the second element.NOTETerminology clash! Inner and outer sequences—The MSDN documentation forthe Join method used to evaluate inner joins unhelpfully calls thesequences involved inner and outer. This has nothing to do with innerjoins and outer joins—it’s just a way of differentiating between thesequences. You can think of them as first and second, left and right, Bertand Ernie—anything you like that helps you. I’ll use left and right for thischapter. Aside from anything else, it makes it obvious which sequence iswhich in diagram form.The two sequences can be anything you like: the right sequence can even be the sameas the left sequence, if that’s useful. (Imagine finding pairs of people who were bornon the same day, for example.) The only thing that matters is that the two key selectorexpressions must result in the same type of key 4 . You can’t join a sequence of people4It is also valid for there to be two key types involved, with an implicit conversion from one to the other. Oneof the types must be a better choice than the other, in the same way that the compiler infers the type of animplicitly typed array. In my experience you rarely need to consciously consider this detail.Licensed to Rhona Hadida