$exists — MongoDB Technical Interview Questions & Answers

Matching documents where a field is present (or absent).

$exists

  • Simple meaning: $exists checks whether a field is present (even if null, depending on value).
  • How to use: db.students.find({ city: { $exists: true } }) students who have a city field. false = field missing.
  • Try this: insert one student without city, one with city, then $exists true/false.
  • Common mistake: using $exists to mean “not empty string”. Empty string still exists.
  • Interview tip: Schema is flexible — $exists is how you find docs that gained a new field.

Technical Interview Tip: Flexible schema → $exists to find who has the field.

Code Examples for $exists

$exists — working example

db.users.find({ phone: { $exists: true } })
db.users.find({ phone: { $exists: false } })

$exists checks whether a field is present (even if null — combine with $ne: null if needed).

How to Explain $exists to an Interviewer

What does $exists do?

Match documents where a field is present or absent