LINQ Pocket Reference

Joining

Method	Description	SQL equivalents
Join	Applies a lookup strategy to match elements from two collections, emitting a flat result set	INNER JOIN
Group Join	As above, but emits a hierarchical result set	INNER JOIN, LEFT OUTER JOIN

Join and GroupJoin

Join arguments

Argument	Type
Outer sequence	IEnumerable<TOuter>
Inner sequence	IEnumerable<TInner>
Outer key selector	TOuter => TKey
Inner key selector	TInner => TKey
Result selector	(TOuter,TInner) => TResult

GroupJoin arguments

Argument	Type
Outer sequence	IEnumerable<TOuter>
Inner sequence	IEnumerable<TInner>
Outer key selector	TOuter => TKey
Inner key selector	TInner => TKey
Result selector	(TOuter,IEnumerable<TInner>) => Tresult

Return type = IEnumerable<TResult>

Comprehension syntax

	from outer-var in outer-enumerable
	join inner-var in inner-enumerable
	  on outer-key-expr equals inner-key-expr
	 [ into identifier ]

Overview

Join and GroupJoin mesh two input sequences into a single output sequence. Join emits flat output; GroupJoin emits hierarchical output.

Join and GroupJoin provide an alternative strategy to Select and SelectMany. The advantage of Join and GroupJoin is that they execute efficiently over local in-memory collections because they first load the inner sequence into a keyed lookup, avoiding the need to repeatedly enumerate over every inner element. Their disadvantage is that they offer the equivalent of inner and left outer joins only; cross joins and non-equi joins must still be done with Select /SelectMany. With LINQ to SQL ...