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- $setDifference (aggregation)
$setDifference (aggregation)¶
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Definition¶
-
$setDifference
¶ New in version 2.6.
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:The arguments can be any valid expression as long as they each resolve to an array. For more information on expressions, see Expressions.
Behavior¶
$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 | |
---|---|---|
{ $setDifference: [ [ "a", "b", "a" ], [ "b", "a" ] ] } |
[ ] |
|
{ $setDifference: [ [ "a", "b" ], [ [ "a", "b" ] ] ] } |
[ "a", "b" ] |
Example¶
Consider an experiments
collection with the following documents:
The following operation uses the $setDifference
operator
to return an array of elements found in the B
array but not
in the A
array:
The operation returns the following results: