Unique index behavior in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a unique index in MongoDB?

A unique index in MongoDB ensures that the indexed field does not have duplicate values across documents in a collection. It helps keep data unique and prevents duplicate entries.

beginner

What happens if you try to insert a document with a duplicate value on a unique indexed field?

MongoDB will reject the insert operation and return an error because the unique index prevents duplicate values in that field.

intermediate

Can a unique index be created on multiple fields in MongoDB?

Yes, MongoDB supports unique compound indexes that enforce uniqueness across the combination of multiple fields together, not just one field.

intermediate

How does MongoDB treat documents with missing fields when a unique index is created on that field?

MongoDB allows multiple documents to have missing or null values for a unique indexed field because null is treated as a unique value for each document.

beginner

How can you create a unique index on the 'email' field in a MongoDB collection named 'users'?

Use the command: db.users.createIndex({ email: 1 }, { unique: true }) to create a unique index on the 'email' field.

What does a unique index in MongoDB prevent?

A Indexes on multiple fields

B Null values in the indexed field

C All documents from being inserted

D Duplicate values in the indexed field

A unique index prevents duplicate values in the indexed field, ensuring each value is unique.

What happens if you insert a document with a duplicate value on a unique indexed field?

A MongoDB returns an error and rejects the insert

B The duplicate is ignored silently

C MongoDB updates the existing document

D The insert succeeds without error

MongoDB rejects the insert and returns an error to maintain uniqueness.

Can a unique index be created on multiple fields in MongoDB?

A No, only single fields can have unique indexes

B Yes, using a compound unique index

C Yes, but only if fields are numeric

D No, MongoDB does not support unique indexes

MongoDB supports compound unique indexes that enforce uniqueness across multiple fields combined.

How does MongoDB treat multiple documents missing a field with a unique index on that field?

A It rejects all but one document

B It treats missing fields as duplicates

C It allows multiple documents with missing or null values

D It converts missing fields to empty strings

MongoDB allows multiple documents to have missing or null values for a unique indexed field.

Which command creates a unique index on the 'username' field in the 'accounts' collection?

A db.accounts.createIndex({ username: 1 }, { unique: true })

B db.accounts.createIndex({ username: 1 })

C db.accounts.ensureIndex({ username: 1 }, true)

D db.accounts.createUniqueIndex({ username: 1 })

The correct syntax to create a unique index is createIndex with the option { unique: true }.

Explain what a unique index is in MongoDB and how it affects data insertion.

Think about how MongoDB keeps certain fields unique across documents.

Describe how MongoDB handles documents with missing or null values when a unique index is applied on that field.

Consider how null values are treated differently than duplicates.