$setDifference (aggregation)

Definition

$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 Expressions.

Behavior

$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 contains 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

Notes

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

[ "c" ]

"a" is present in both arrays, so it is removed from the result. "c" is only present in the first array.

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

[ ]

Both "a" and "c" are present in both arrays so they are removed from the result. No more elements remain in the first array.

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

[ ]

Both "a" and "b" are present in both arrays so they are removed from the result. Duplicates are also removed. No more elements remain in the first array.

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

[ "a", "b" ]

The strings "a" and "b" from the first array are not found in the second array, which contains a nested array.

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

[ ]

The first array is empty so there are no elements to subtract from, regardless of what's in the second array.

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

[ ]

"a" is present in the second array so it is removed from the result. Duplicates are also removed. No more elements remain in the first array.

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 $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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$second

$setEquals