LINQ Pocket Reference

Set Operators

Method	Description	SQL equivalents
Concat	Returns a concatenation of elements in each of the two sequences	UNION ALL
Union	Returns a concatenation of elements in each of the two sequences, excluding duplicates	UNION
Intersect	Returns elements present in both sequences	WHERE…IN…
Except	Returns elements present in the first, but not the second sequence	EXCEPT or WHERE…NOT IN…

Concat and Union

Contact returns all the elements of the first sequence, followed by all the elements of the second. Union does the same, but removes any duplicates:

	int[] seq1 = { 1, 2, 3 }, seq2 = { 3, 4,5 };

	IEnumerable<int>
	  concat = seq1.Concat (seq2), // { 1, 2,3, 3, 4, 5 }
	  union  = seq1.Union  (seq2); // { 1, 2, 3,4, 5 }

Intersect and Except

Intersect returns the elements that two sequences have in common. Except returns the elements in the first input sequence that are not present in the second:

	int[] seq1 = { 1, 2, 3 }, seq2 = { 3, 4,5 };

	IEnumerable<int>
	  commonality = seq1.Intersect (seq2),   // { 3 }
	  difference1 = seq1.Except    (seq2),   // {1, 2 }
	  difference2 = seq2.Except    (seq1);   // {4, 5 }

Enumerable.Except works internally by loading all of the elements in the first collection into a dictionary, then removing from the dictionary all elements present in the second sequence. The equivalent in SQL is a NOT EXISTS or NOT IN subquery:

	SELECT number FROM numbers1Table
	WHERE number NOT IN (SELECT number FROM numbers2Table)