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$setIntersection (aggregation)

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$setIntersection

Takes two or more arrays and returns an array that contains the elements that appear in every input array.

$setIntersection has the following syntax:

{ $setIntersection: [ <array1>, <array2>, ... ] }

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

$setIntersection performs set operation on arrays, treating arrays as sets. If an array contains duplicate entries, $setIntersection ignores the duplicate entries. $setIntersection ignores the order of the elements.

$setIntersection 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 no intersections are found (i.e. the input arrays contain no common elements), $setIntersection returns an empty array.

If a set contains a nested array element, $setIntersection does not descend into the nested array but evaluates the array at top-level.

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

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 $setIntersection operator to return an array of elements common to both the A array and the B array:

db.experiments.aggregate(
[
{ $project: { A: 1, B: 1, commonToBoth: { $setIntersection: [ "$A", "$B" ] }, _id: 0 } }
]
)

The operation returns the following results:

{ "A" : [ "red", "blue" ], "B" : [ "red", "blue" ], "commonToBoth" : [ "blue", "red" ] }
{ "A" : [ "red", "blue" ], "B" : [ "blue", "red", "blue" ], "commonToBoth" : [ "blue", "red" ] }
{ "A" : [ "red", "blue" ], "B" : [ "red", "blue", "green" ], "commonToBoth" : [ "blue", "red" ] }
{ "A" : [ "red", "blue" ], "B" : [ "green", "red" ], "commonToBoth" : [ "red" ] }
{ "A" : [ "red", "blue" ], "B" : [ ], "commonToBoth" : [ ] }
{ "A" : [ "red", "blue" ], "B" : [ [ "red" ], [ "blue" ] ], "commonToBoth" : [ ] }
{ "A" : [ "red", "blue" ], "B" : [ [ "red", "blue" ] ], "commonToBoth" : [ ] }
{ "A" : [ ], "B" : [ ], "commonToBoth" : [ ] }
{ "A" : [ ], "B" : [ "red" ], "commonToBoth" : [ ] }
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