MongoDB $setIntersection Operator

What Is $setIntersection?

$setIntersection is a MongoDB aggregation expression that returns the unique elements shared by every input array. Arrays are treated as sets—duplicates are ignored; only the overlap remains.

$setIntersection = values in ALL arrays

{ $setIntersection: [ <array1>, <array2>, ... ] }

[1, 2, 3] ∩ [2, 3, 4] → [2, 3]
["a","a","b"] ∩ ["b"] → ["b"]
no overlap → []

duplicates ignored
null / missing arg → null

vs $setUnion → combine unique values

How to Use $setIntersection in MongoDB

Pass two or more arrays and keep only the values that appear in every one.

1. Choose an expression stage ($addFields or $project).
2. Pass the arrays to compare (fields and/or literals).
3. Read the result array (empty [] means no overlap).
4. Use $setUnion when you want the combined unique list instead.

Pseudocode

for each document:
    arrays = evaluate each argument
    if any array is null or missing:
        result = null
    else:
        result = unique values present in every array
    emit { ..., field: result }

Cheat Sheet

Keep these facts handy while finding shared array values.

Live Preview

Intersect two skill lists and see the shared values (set rules).

a = ["math", "quiz", "art"]
b = ["quiz", "math", "science"]

$setIntersection → ["math", "quiz"]

Worked Walkthrough — Shared Skills

Asha has skills math, quiz, art. The course requires quiz, math, science. The overlap is math and quiz.

skills   = ["math", "quiz", "art"]
required = ["quiz", "math", "science"]

math     → in both ✓
quiz     → in both ✓
art      → only skills ✗
science  → only required ✗

$setIntersection → ["math", "quiz"]
  (order follows first array)

MongoDB Examples

We use one student with a skills list and a required list. First insert the sample, then compute the intersection, then contrast with $setUnion. Run these in mongosh after inserting the sample data once.

db.students.insertOne({
  _id: 1,
  name: "Asha",
  skills: ["math", "quiz", "art"],
  required: ["quiz", "math", "science"]
});

db.students.find()

{
  _id: 1,
  name: "Asha",
  skills: ["math", "quiz", "art"],
  required: ["quiz", "math", "science"]
}

skills:   math  quiz  art
required: quiz  math  science
                ↓
shared:   math  quiz

db.students.aggregate([
  {
    $addFields: {
      shared: {
        $setIntersection: ["$skills", "$required"]
      }
    }
  }
]);

{
  _id: 1,
  name: "Asha",
  skills: ["math", "quiz", "art"],
  required: ["quiz", "math", "science"],
  shared: ["math", "quiz"]
}

db.students.aggregate([
  {
    $addFields: {
      shared: { $setIntersection: ["$skills", "$required"] },
      allUnique: { $setUnion: ["$skills", "$required"] }
    }
  }
]);

{
  _id: 1,
  name: "Asha",
  skills: ["math", "quiz", "art"],
  required: ["quiz", "math", "science"],
  shared: ["math", "quiz"],
  allUnique: ["math", "quiz", "art", "science"]
}

Performance Notes

Intersecting short tag lists is inexpensive. Filter early with $match when only some documents need the check.

Key Takeaways