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Aggregation Pipeline Operators

$setUnion (aggregation)

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  • Definition
  • Behavior
  • Example

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.

Example
Result
{ $setUnion: [ [ "a", "b", "a" ], [ "b", "a" ] ] }
[ "b", "a" ]
{ $setUnion: [ [ "a", "b" ], [ [ "a", "b" ] ] ] }
[ [ "a", "b" ], "b", "a" ]

Note

Starting in MongoDB 5.2, the sort order for $setUnion is undefined. To sort an array, refer to $sortArray.

Example

Consider an flowers collection with the following documents:

db.flowers.insertMany( [
   { "_id" : 1, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "rose", "orchid" ] },
   { "_id" : 2, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "orchid", "rose", "orchid" ] },
   { "_id" : 3, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "rose", "orchid", "jasmine" ] },
   { "_id" : 4, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ "jasmine", "rose" ] },
   { "_id" : 5, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ ] },
   { "_id" : 6, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ [ "rose" ], [ "orchid" ] ] },
   { "_id" : 7, "flowerFieldA" : [ "rose", "orchid" ], "flowerFieldB" : [ [ "rose", "orchid" ] ] },
   { "_id" : 8, "flowerFieldA" : [ ], "flowerFieldB" : [ ] },
   { "_id" : 9, "flowerFieldA" : [ ], "flowerFieldB" : [ "rose" ] }
] )

The following operation uses the $setUnion operator to return an array of elements found in the flowerFieldA array or the flowerFieldB array or both:

db.flowers.aggregate(
   [
     { $project: { flowerFieldA:1, flowerFieldB: 1, allValues: { $setUnion: [ "$flowerFieldA", "$flowerFieldB" ] }, _id: 0 } }
   ]
)

The operation returns the following results:

{ "flowerFieldA": [ "rose", "orchid" ], "flowerFieldB": [ "rose", "orchid" ], "allValues": [ "orchid", "rose" ] }
{ "flowerFieldA": [ "rose", "orchid" ], "flowerFieldB": [ "orchid", "rose", "orchid" ], "allValues": [ "orchid", "rose" ] }
{ "flowerFieldA": [ "rose", "orchid" ], "flowerFieldB": [ "rose", "orchid", "jasmine" ], "allValues": [ "orchid", "rose", "jasmine" ] }
{ "flowerFieldA": [ "rose", "orchid" ], "flowerFieldB": [ "jasmine", "rose" ], "allValues": [ "orchid", "rose", "jasmine" ] }
{ "flowerFieldA": [ "rose", "orchid" ], "flowerFieldB": [ ], "allValues": [ "orchid", "rose" ] }
{ "flowerFieldA": [ "rose", "orchid" ], "flowerFieldB": [ [ "rose" ], [ "orchid" ] ], "allValues": [ "orchid", "rose", [ "rose" ], [ "orchid" ] ] }
{ "flowerFieldA": [ "rose", "orchid" ], "flowerFieldB": [ [ "rose", "orchid" ] ], "allValues": [ "orchid", "rose", [ "rose", "orchid" ] ] }
{ "flowerFieldA": [ ], "flowerFieldB": [ ], "allValues": [ ] }
{ "flowerFieldA": [ ], "flowerFieldB": [ "rose" ], "allValues": [ "rose" ] }

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$setIsSubset (aggregation)