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MongoDB Aggregation Set Operator - $setDifference

MongoDB: $setDifference

The MongoDB $setDifference operators takes exactly two argument expressions i.e. two sets and returns an array containing the elements that only exist in the first set; i.e. performs a relative complement of the second set relative to the first.

Syntax :

{ $setDifference: [ <expression1>, <expression2> ] }
  • The $setDifference operator can only performs set operation on arrays and treated as sets. If any duplicate value contains in an array, it ignores by $setDifference 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 $setDifference operation.
  • When a set contains a nested array element, the $setDifference 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 : $setDifference

The following aggregation operation uses the $setDifference operator compares the array A and B to return an array of elements found in the B array but not in the A array:

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

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

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

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