/ /

/ /

$setUnion

Definition

Takes two or more arrays and returns a single array containing the unique elements that appear in any input array. $setUnion can be used as an aggregation accumulator or an array operator.

Aggregation Accumulator

$setUnion

$setUnion is available as an accumulator in these stages:

Syntax

When used as an aggregation accumulator, $setUnion has the following syntax:

{ $setUnion: "<array field>" }

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 contains 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.

Arguments that are null or resolve to null are included in the results. Arguments that refer to a field that is missing are not included in the results.

Example

Create a collection named sales with the following documents:

db.sales.insertMany( [
   {
     _id: 1,
     items: [ "laptop", "tablet" ],
     location: "NYC"
   },
   {
     _id: 2,
     items: [ "phone", "tablet" ],
     location: "NYC"
   },
   {
     _id: 3,
     location: "NYC"
   },
   {
     _id: 4,
     items: [ "desktop", { "accessories": [ "mouse", "keyboard"] } ],
     location: "NYC"
   }
] )

This example shows how you can use $setUnion as an accumulator. This example accumulates all the unique elements in the items arrays when grouping on the location field:

db.sales.aggregate( [
   {
      $group: {
         _id: "$location",
         array: { "$setUnion": "$items" }
      }
   }
] )

The operation returns the following result:

[
   {
      "_id": "NYC",
      "array": [
          "laptop", "tablet", "phone", "desktop", { "accessories": [ "mouse", "keyboard"] }
      ]
   }
]

Array Operator

$setUnion

Syntax

When used as an array operator, $setUnion has the following syntax:

{
   $setUnion: [ <expression1>, <expression2>, ... ]
}

Behavior

$setUnion filters out duplicates in its result to output an array that contains only unique entries. The order of the elements in the output array is unspecified.

The arguments can be any valid expression as long as they each resolve to an array. For more information on expressions, see Expressions.

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

Notes

{ $setUnion: [
  [ "a", "c" ],
  [ "a", "b" ]
] }

[ "a", "c", "b" ]

All elements from both arrays are combined and duplicate elements removed.

{ $setUnion: [
  [ "a", "c" ],
  [ "a", "b", "c" ]
] }

[ "a", "c", "b" ]

All elements from both arrays are combined and duplicate elements removed.

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

[ "a", "b" ]

All elements from both arrays are combined and duplicate elements removed.

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

[ "a", "b", [ "a", "b" ] ]

The result includes "a" and "b" as individual string elements and the array [ "a", "b" ] as a unique element.

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

[ "a", "b" ]

The result consists of the unique elements in the second array because the first array is empty.

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

[ "a", "b" ]

All elements from both arrays are combined and duplicate elements removed.

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" ] }

Limitations

$setUnion only supports arrays and expressions that resolve to an array.
$setUnion does not guarantee the order of the elements in the output array.

Learn More

Back

$setIsSubset

$shift