Delete all documents in collection in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What command deletes all documents in a MongoDB collection?

Use db.collection.deleteMany({}) to delete all documents in the collection.

beginner

What does the empty filter {} mean in deleteMany({})?

It means "match all documents" so every document in the collection will be deleted.

intermediate

How is deleteMany({}) different from drop() in MongoDB?

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

intermediate

What happens if you run db.collection.deleteOne({}) with an empty filter?

It deletes only one document that matches the empty filter, so effectively it deletes one random document.

beginner

Why should you be careful when using deleteMany({})?

Because it deletes all documents in the collection, which can cause data loss if done unintentionally.

Which MongoDB command deletes all documents in a collection but keeps the collection itself?

A db.collection.drop()

B db.collection.deleteMany({})

C db.collection.removeOne({})

D db.collection.clear()

deleteMany({}) deletes all documents but keeps the collection. drop() deletes the entire collection.

What does the filter {} mean when used in deleteMany({})?

A Delete all documents

B Delete no documents

C Delete documents with empty fields

D Delete documents with null values

An empty filter {} matches all documents, so all documents are deleted.

What is the effect of db.collection.deleteOne({})?

A Deletes no documents

B Deletes all documents

C Deletes the collection

D Deletes one document

deleteOne({}) deletes one document matching the filter, here the empty filter matches any document.

Which command removes the entire collection including its indexes?

A db.collection.deleteMany({})

B db.collection.deleteOne({})

C db.collection.drop()

D db.collection.removeAll()

drop() deletes the whole collection and its indexes.

Why should you be cautious when using deleteMany({})?

A It can delete all documents causing data loss

B It can delete the collection

C It does not delete any documents

D It can delete only one document

deleteMany({}) deletes all documents, which can cause data loss if used by mistake.

Explain how to delete all documents in a MongoDB collection without deleting the collection itself.

Think about the command that removes documents but not the collection.

Describe the difference between deleteMany({}) and drop() in MongoDB.

One removes data, the other removes the whole container.

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