Unique index behavior in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What happens when inserting duplicate values on a unique index?

Consider a MongoDB collection with a unique index on the field email. What will be the result of inserting a document with an email value that already exists in the collection?

MongoDB

db.users.createIndex({ email: 1 }, { unique: true });
db.users.insertOne({ name: "Alice", email: "[e-mail]" });
db.users.insertOne({ name: "Bob", email: "[e-mail]" });

A The second insert will fail with a duplicate key error.

B The second insert will be ignored silently.

C The second insert will succeed and create a duplicate email.

D The second insert will overwrite the first document.

💡 Hint

Unique indexes prevent duplicate values in the indexed field.

A unique index in MongoDB enforces uniqueness for the indexed field. Attempting to insert a document with a duplicate value in that field causes a duplicate key error.

❓ query_result

intermediate

What is the output of this query with a unique sparse index?

A collection has a unique sparse index on the field phone. The collection contains documents with and without the phone field. What will be the result of inserting two documents without the phone field?

MongoDB

db.contacts.createIndex({ phone: 1 }, { unique: true, sparse: true });
db.contacts.insertOne({ name: "John" });
db.contacts.insertOne({ name: "Jane" });

A Both inserts fail because the unique index requires all documents to have the field.

B The second insert fails with a duplicate key error.

C Both inserts succeed because sparse indexes ignore documents missing the indexed field.

D Only the first insert succeeds; the second is ignored.

💡 Hint

Sparse indexes only index documents that contain the indexed field.

A unique sparse index only enforces uniqueness on documents that have the indexed field. Documents missing the field are not indexed and can be duplicated.

📝 Syntax

advanced

Which command correctly creates a unique compound index on fields username and email?

Select the valid MongoDB command to create a unique compound index on username and email fields.

A db.users.createIndex(["username", "email"], { unique: true });

B db.users.createIndex({ username: 1, email: 1 }, { unique: true });

C db.users.createIndex({ username: 1 }, { email: 1, unique: true });

D db.users.createIndex({ username: 1, email: 1 }, unique = true);

💡 Hint

The index keys must be in an object with field names and sort order.

The correct syntax uses an object with field names and sort order, and the options object with unique: true.

🔧 Debug

advanced

Why does this unique index creation fail?

You try to create a unique index on the username field, but MongoDB returns an error. The collection already has documents with duplicate username values. What is the cause of the failure?

MongoDB

db.users.createIndex({ username: 1 }, { unique: true });

A Unique indexes cannot be created on string fields.

B The syntax of createIndex is incorrect.

C The database is in read-only mode.

D The collection contains duplicate usernames, violating the unique index constraint.

💡 Hint

Unique indexes require all existing data to be unique on the indexed field.

MongoDB cannot create a unique index if existing documents violate uniqueness. The duplicates must be removed or fixed first.

🧠 Conceptual

expert

How does a unique partial index differ from a unique sparse index in MongoDB?

Choose the correct statement about the difference between a unique partial index and a unique sparse index.

A A unique partial index enforces uniqueness only on documents matching a filter expression; a unique sparse index ignores documents missing the indexed field.

B A unique sparse index enforces uniqueness only on documents matching a filter expression; a unique partial index ignores documents missing the indexed field.

C Both indexes behave the same and enforce uniqueness on all documents regardless of fields.

D Unique partial indexes allow duplicates; unique sparse indexes do not.

💡 Hint

Partial indexes use a filter expression; sparse indexes depend on field presence.

Partial indexes apply a filter to select documents to index, enforcing uniqueness only on those. Sparse indexes index only documents with the indexed field present.