$setDifference (Set Difference)

Last updated: 2026-10-09

$setDifference (Set Difference)

$setDifference returns elements present in the first array but absent from the second, with duplicates removed. Typical uses: finding products a user bought but did not favorite, or diffing two name lists. Both arguments must be arrays, and null elements are ignored. The result has no guaranteed order, and relative positions after deduplication are not preserved. Set operators treat arrays as sets, forming a family with $setIntersection and $setUnion.

📝 Syntax

{ $setDifference: [ <arrayA>, <arrayB> ] }

⚙️ Parameters

Returns：An array (deduplicated) of elements in A but not in B; null when either argument is null

▶ Example

Run the commands below in your local mongosh (requires a running mongod):

// $setDifference: bought but not favorited
db.users.aggregate([
  { $project: {
      notFav: { $setDifference: ['$bought', '$favorites'] }
  } }
])

Output：

{ '_id': 1, 'notFav': [ 'skuB' ] }

📌 Notes

• The result is deduplicated with no guaranteed order; never rely on positions
• null elements are ignored and do not affect the result
• Both arguments must be arrays or the operator throws

🏷️ Related Tags

💡 Related Cases

📚 Related Tutorials

❓ FAQ

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