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$setDifference (aggregation)

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  • Definition
  • Behavior
  • Example
$setDifference

Takes 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.

$setDifference has the following syntax:

{ $setDifference: [ <expression1>, <expression2> ] }

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

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

$setDifference 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, $setDifference does not descend into the nested array but evaluates the array at top-level.

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

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 $setDifference operator to return an array of elements found in the flowerFieldB array but not in the flowerFieldA array:

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

The operation returns the following results:

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

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