$setUnion (aggregation)
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Definition
$setUnion
Takes two or more arrays and returns an array containing the elements that appear in any input array.
$setUnion
has the following syntax:{ $setUnion: [ <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
$setUnion
performs set operation on arrays, treating arrays
as sets. If an array contains duplicate entries, $setUnion
ignores the duplicate entries. $setUnion
ignores the order of
the elements.
$setUnion
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, $setUnion
does not descend
into the nested array but evaluates the array at top-level.
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Note
Starting in MongoDB 5.2, the sort order for $setUnion
is undefined. To sort an array, refer to $sortArray
.
Example
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 $setUnion
operator to
return an array of elements found in the A
array or the
B
array or both:
db.experiments.aggregate( [ { $project: { A:1, B: 1, allValues: { $setUnion: [ "$A", "$B" ] }, _id: 0 } } ] )
The operation returns the following results:
{ "A": [ "red", "blue" ], "B": [ "red", "blue" ], "allValues": [ "blue", "red" ] } { "A": [ "red", "blue" ], "B": [ "blue", "red", "blue" ], "allValues": [ "blue", "red" ] } { "A": [ "red", "blue" ], "B": [ "red", "blue", "green" ], "allValues": [ "blue", "red", "green" ] } { "A": [ "red", "blue" ], "B": [ "green", "red" ], "allValues": [ "blue", "red", "green" ] } { "A": [ "red", "blue" ], "B": [ ], "allValues": [ "blue", "red" ] } { "A": [ "red", "blue" ], "B": [ [ "red" ], [ "blue" ] ], "allValues": [ "blue", "red", [ "red" ], [ "blue" ] ] } { "A": [ "red", "blue" ], "B": [ [ "red", "blue" ] ], "allValues": [ "blue", "red", [ "red", "blue" ] ] } { "A": [ ], "B": [ ], "allValues": [ ] } { "A": [ ], "B": [ "red" ], "allValues": [ "red" ] }