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Aggregation Pipeline Operators

$setDifference (aggregation)

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  • Definition
  • Behavior
  • Example

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 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" ]

Example

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 $setDifference operator to return an array of elements found in the B array but not in the A array:

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

The operation returns the following results:

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

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$second (aggregation)