Delete all documents in collection in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of deleting all documents using deleteMany({})?

Consider a MongoDB collection named users with 5 documents. What will be the count of documents after running db.users.deleteMany({})?

MongoDB

db.users.deleteMany({})
db.users.countDocuments()

A 0

B 5

C null

D 1

💡 Hint

deleteMany with an empty filter removes all documents.

The deleteMany({}) command deletes all documents in the collection. After deletion, countDocuments() returns 0.

📝 Syntax

intermediate

Which command correctly deletes all documents in a MongoDB collection?

Choose the correct MongoDB command to delete all documents from the orders collection.

A db.orders.remove()

B db.orders.deleteMany({})

C db.orders.deleteAll()

D db.orders.delete({})

💡 Hint

The method deleteMany requires a filter argument.

The correct method to delete all documents is deleteMany with an empty filter object. remove() requires a filter or is deprecated. deleteAll() and delete() do not exist.

❓ optimization

advanced

Which approach is more efficient to remove all documents from a large MongoDB collection?

You want to clear a large collection named logs. Which option is the most efficient?

A db.logs.drop()

B db.logs.deleteMany({})

C db.logs.remove({})

D db.logs.deleteOne({})

💡 Hint

Dropping a collection removes all documents and the collection itself.

Dropping the collection removes all documents and frees storage immediately. deleteMany removes documents but keeps the collection and indexes.

🔧 Debug

advanced

Why does db.products.deleteMany() cause an error?

A developer runs db.products.deleteMany() and gets an error. What is the cause?

MongoDB

db.products.deleteMany()

A Syntax error due to missing semicolon

B deleteMany is not a function

C Collection products does not exist

D Missing filter argument causes a TypeError

💡 Hint

deleteMany requires a filter argument even if empty.

deleteMany requires a filter argument. Calling it without arguments causes a TypeError.

🧠 Conceptual

expert

What happens to indexes when you delete all documents with deleteMany({})?

If you run db.inventory.deleteMany({}) on a collection with indexes, what happens to those indexes?

A Indexes become corrupted and unusable

B All indexes are dropped automatically

C Indexes remain intact and usable

D Indexes are converted to unique constraints

💡 Hint

Deleting documents does not affect the structure of the collection.

deleteMany removes documents but does not drop or alter indexes. Indexes remain intact and ready for new data.

Practice

1. What does the MongoDB command db.collection.deleteMany({}) do?

easy

A. Deletes all documents in the collection but keeps the collection itself.

B. Deletes the entire collection including its structure.

C. Deletes only the first document in the collection.

D. Updates all documents in the collection to empty values.

Solution

1. Step 1: Understand the deleteMany method

   The deleteMany method removes documents matching the filter. An empty filter {} matches all documents.

2. Step 2: Effect of using an empty filter

   Using {} deletes all documents but does not remove the collection itself.

3. Final Answer:

   Deletes all documents in the collection but keeps the collection itself. -> Option A

4. Quick Check:

   deleteMany({}) removes all documents [OK]

Hint: Empty filter {} deletes all documents, collection stays [OK]

Common Mistakes:

• Thinking deleteMany({}) drops the collection
• Confusing deleteMany with deleteOne
• Assuming documents are updated, not deleted

2. Which of the following is the correct syntax to delete all documents from a MongoDB collection named users?

easy

A. db.users.deleteAll({})

B. db.users.deleteMany()

C. db.users.deleteMany({})

D. db.users.removeAll()

Solution

1. Step 1: Identify the correct method and syntax

   The method to delete multiple documents is deleteMany, which requires a filter argument.

2. Step 2: Use an empty filter to delete all documents

   Passing an empty object {} as the filter deletes all documents in the collection.

3. Final Answer:

   db.users.deleteMany({}) -> Option C

4. Quick Check:

   Correct syntax includes empty filter {} [OK]

Hint: deleteMany needs a filter; use {} for all documents [OK]

Common Mistakes:

• Omitting the filter argument in deleteMany
• Using non-existent methods like deleteAll or removeAll
• Using deleteMany without parentheses

3. Given the collection products with 5 documents, what will be the result of running db.products.deleteMany({}) followed by db.products.find().toArray()?

medium

A. Returns an empty array [] because all documents are deleted.

B. Returns all 5 documents because deleteMany does not delete anything.

C. Returns an error because deleteMany requires a filter.

D. Returns only the first document because deleteMany deletes one document.

Solution

1. Step 1: Effect of deleteMany({}) on documents

   Using deleteMany({}) deletes all documents in the collection.

2. Step 2: Result of find() after deletion

   After deletion, find() returns no documents, so toArray() returns an empty array.

3. Final Answer:

   Returns an empty array [] because all documents are deleted. -> Option A

4. Quick Check:

   deleteMany({}) empties collection, find() returns [] [OK]

Hint: deleteMany({}) clears all documents; find() shows empty [OK]

Common Mistakes:

• Expecting documents to remain after deleteMany({})
• Thinking deleteMany deletes only one document
• Assuming deleteMany without filter causes error

4. You run db.orders.deleteMany() to delete all documents in the orders collection but get an error. What is the likely cause?

medium

A. The collection orders does not exist.

B. You need to use removeMany instead.

C. deleteMany cannot delete multiple documents.

D. Missing filter argument in deleteMany method.

Solution

1. Step 1: Check deleteMany method requirements

   The deleteMany method requires a filter argument; omitting it causes a syntax error.

2. Step 2: Confirm correct usage to delete all documents

   To delete all documents, pass an empty filter {} as argument.

3. Final Answer:

   Missing filter argument in deleteMany method. -> Option D

4. Quick Check:

   deleteMany needs a filter argument [OK]

Hint: Always provide filter to deleteMany, use {} for all [OK]

Common Mistakes:

• Calling deleteMany without parentheses or filter
• Confusing deleteMany with removeMany (not a method)
• Assuming collection absence causes this error

5. You want to clear all documents from the logs collection but keep the collection and its indexes intact. Which command should you use?

hard

A. db.logs.deleteOne({})

B. db.logs.deleteMany({})

C. db.logs.removeAll()

D. db.logs.drop()

Solution

1. Step 1: Understand difference between drop and deleteMany

   drop() removes the entire collection and indexes, while deleteMany({}) removes all documents but keeps collection and indexes.

2. Step 2: Choose command that clears documents but preserves collection

   Using deleteMany({}) clears all documents without dropping the collection or indexes.

3. Final Answer:

   db.logs.deleteMany({}) -> Option B

4. Quick Check:

   deleteMany({}) clears docs, keeps collection [OK]

Hint: Use deleteMany({}) to clear data, drop() removes collection [OK]

Common Mistakes:

• Using drop() which deletes collection and indexes
• Using non-existent removeAll() method
• Using deleteOne() which deletes only one document