MongoDB Aggregation Set Operator - $setIntersection

MongoDB: $setIntersection

The MongoDB $setIntersection operators takes two or more arrays and returns a set of array with elements that appear in all of the input sets.

Syntax:

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

• The $setIntersection operator can only performs set operation on arrays and treated as sets. If any duplicate value contains in an array, it ignores by $setIntersection operator. The order of the elements in not important in this operation..
• Only the unique value in an array appears as a result after filtering the duplicates values in the operation.
• When a set contains a nested array element, the $setIntersection does not descend into the nested array but evaluates the array at top-level.

Sample collection test_collection

{ "_id" : 1, "A" : [ "cat", "rat" ], "B" : [ "cat", "rat" ] }
{ "_id" : 2, "A" : [ "cat", "rat" ], "B" : [ ] }
{ "_id" : 3, "A" : [ ], "B" : [ "cat" ] }
{ "_id" : 4, "A" : [ "cat", "rat" ], "B" : [ "rat", "cat", "rat" ] }
{ "_id" : 5, "A" : [ "cat", "rat" ], "B" : [ "cat", "rat", "dog" ] }
{ "_id" : 6, "A" : [ "cat", "rat" ], "B" : [ [ "cat", "rat" ] ] }
{ "_id" : 7, "A" : [ "cat", "rat" ], "B" : [ [ "cat" ], [ "rat" ] ] }
{ "_id" : 8, "A" : [ ], "B" : [ ] }
{ "_id" : 9, "A" : [ "cat", "rat" ], "B" : [ "dog", "cat" ] }
}

Example: $setIntersection

The following aggregation operation uses the $setIntersection operator compares the array A and B returns an array set that contain the common array elements.

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

After the operation, following result will be returned by the $setIntersection operator.

> db.test_collection.aggregate(
...    [
...      { $project: { A: 1, B: 1, commonToBoth: { $setIntersection: [ "$A", "$B" ] }, _id: 0 } }
...    ]
... );
{ "A" : [ "cat", "rat" ], "B" : [ "cat", "rat" ], "commonToBoth" : [ "cat", "rat" ] }
{ "A" : [ "cat", "rat" ], "B" : [ ], "commonToBoth" : [ ] }
{ "A" : [ ], "B" : [ "cat" ], "commonToBoth" : [ ] }
{ "A" : [ "cat", "rat" ], "B" : [ "rat", "cat", "rat" ], "commonToBoth" : [ "cat", "rat" ] }
{ "A" : [ "cat", "rat" ], "B" : [ "cat", "rat", "dog" ], "commonToBoth" : [ "cat", "rat" ] }
{ "A" : [ "cat", "rat" ], "B" : [ [ "cat", "rat" ] ], "commonToBoth" : [ ] }
{ "A" : [ "cat", "rat" ], "B" : [ [ "cat" ], [ "rat" ] ], "commonToBoth" : [ ] }
{ "A" : [ ], "B" : [ ], "commonToBoth" : [ ] }
{ "A" : [ "cat", "rat" ], "B" : [ "dog", "cat" ], "commonToBoth" : [ "cat" ] }

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