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$setDifference (aggregation)![](/docs/v5.2/assets/link.svg)
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Definition![](/docs/v5.2/assets/link.svg)
$setDifference
Takes two sets and returns an array containing the elements that only exist in the first set; i.e. performs a relative complement of the second set relative to the first.
$setDifference
has the following syntax:{ $setDifference: [ <expression1>, <expression2> ] } The arguments can be any valid expression as long as they each resolve to an array. For more information on expressions, see Expressions.
Behavior![](/docs/v5.2/assets/link.svg)
$setDifference
performs set operation on arrays, treating arrays
as sets. If an array contains duplicate entries, $setDifference
ignores the duplicate entries. $setDifference
ignores the order of
the elements.
$setDifference
filters out duplicates in its result to output an
array that contain only unique entries. The order of the elements in
the output array is unspecified.
If a set contains a nested array element, $setDifference
does not descend
into the nested array but evaluates the array at top-level.
Example | Result | ||
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Example![](/docs/v5.2/assets/link.svg)
Consider an experiments
collection with the following documents:
{ "_id" : 1, "A" : [ "red", "blue" ], "B" : [ "red", "blue" ] } { "_id" : 2, "A" : [ "red", "blue" ], "B" : [ "blue", "red", "blue" ] } { "_id" : 3, "A" : [ "red", "blue" ], "B" : [ "red", "blue", "green" ] } { "_id" : 4, "A" : [ "red", "blue" ], "B" : [ "green", "red" ] } { "_id" : 5, "A" : [ "red", "blue" ], "B" : [ ] } { "_id" : 6, "A" : [ "red", "blue" ], "B" : [ [ "red" ], [ "blue" ] ] } { "_id" : 7, "A" : [ "red", "blue" ], "B" : [ [ "red", "blue" ] ] } { "_id" : 8, "A" : [ ], "B" : [ ] } { "_id" : 9, "A" : [ ], "B" : [ "red" ] }
The following operation uses the $setDifference
operator
to return an array of elements found in the B
array but not
in the A
array:
db.experiments.aggregate( [ { $project: { A: 1, B: 1, inBOnly: { $setDifference: [ "$B", "$A" ] }, _id: 0 } } ] )
The operation returns the following results:
{ "A" : [ "red", "blue" ], "B" : [ "red", "blue" ], "inBOnly" : [ ] } { "A" : [ "red", "blue" ], "B" : [ "blue", "red", "blue" ], "inBOnly" : [ ] } { "A" : [ "red", "blue" ], "B" : [ "red", "blue", "green" ], "inBOnly" : [ "green" ] } { "A" : [ "red", "blue" ], "B" : [ "green", "red" ], "inBOnly" : [ "green" ] } { "A" : [ "red", "blue" ], "B" : [ ], "inBOnly" : [ ] } { "A" : [ "red", "blue" ], "B" : [ [ "red" ], [ "blue" ] ], "inBOnly" : [ [ "red" ], [ "blue" ] ] } { "A" : [ "red", "blue" ], "B" : [ [ "red", "blue" ] ], "inBOnly" : [ [ "red", "blue" ] ] } { "A" : [ ], "B" : [ ], "inBOnly" : [ ] } { "A" : [ ], "B" : [ "red" ], "inBOnly" : [ "red" ] }