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LINQ Pocket Reference by Joseph Albahari, Ben Albahari

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Subqueries

A subquery is a query contained within another query’s lambda expression. The following example uses a subquery to sort musicians by their last name:

	string[] musos =
	  { "David Gilmour","Roger Waters", "Rick Wright" };

	IEnumerable<string> query =
	  musos.OrderBy (m => m.Split().Last());

m.Split converts each string into a collection of words, upon which we then call the Last query operator. Last is the subquery; query references the outer query.

Subqueries are permitted because you can put any valid C# expression on the right side of a lambda. A subquery is simply another C# expression, meaning that the rules for subqueries are a consequence of the rules for lambda expressions (and the behavior of query operators in general).

A subquery is privately scoped to the enclosing expression and is able to reference the outer lambda argument (or iteration variable in comprehension syntax).

Last is a very simple subquery. The next query retrieves all strings in an array whose length matches that of the shortest string:

	string[] names = { "Tom","Dick","Harry","Mary","Jay" };

	IEnumerable<string> outerQuery = names
	  .Where (n => n.Length ==
	     names.OrderBy (n2 => n2.Length)
	          .Select  (n2 => n2.Length).First( )
	  );

	// RESULT: Tom, Jay

Here’s the same thing in comprehension syntax:

	IEnumerable<string> comprehension =
	  from     n in names
	  where    n.Length ==
	    (from n2 in names
	     orderby n2.Length
	     select n2.Length).First( )
	  select n;

Because the outer iteration variable (n) is in scope for a subquery, we cannot ...

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