Drop collection vs deleteMany in MongoDB - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Difference in effect between drop() and deleteMany()

What is the main difference between using db.collection.drop() and db.collection.deleteMany({}) in MongoDB?

A drop() only deletes documents matching a filter, deleteMany({}) deletes the entire collection.

B drop() and deleteMany({}) both remove all documents but keep the collection structure intact.

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

D drop() removes documents one by one, deleteMany({}) removes the collection instantly.

💡 Hint

Think about what happens to the collection itself after each operation.

drop() deletes the entire collection and its indexes from the database. deleteMany({}) deletes all documents but leaves the collection and its indexes intact.

❓ query_result

intermediate

Result of drop() on a non-existent collection

What is the result of running db.nonexistent.drop() if the collection does not exist?

A Returns false and does not throw an error.

B Throws an error indicating the collection does not exist.

C Creates the collection and then drops it, returning true.

D Returns true even though the collection did not exist.

💡 Hint

Consider if drop() requires the collection to exist to succeed.

Calling drop() on a non-existent collection returns false and does not throw an error.

📝 Syntax

advanced

Correct syntax to delete all documents in a collection

Which of the following MongoDB commands correctly deletes all documents from the users collection without dropping it?

A db.users.deleteMany({})

B db.users.drop()

C db.users.remove()

D db.users.delete()

💡 Hint

Remember the modern method to delete multiple documents.

deleteMany({}) deletes all documents matching the empty filter, effectively deleting all documents but keeping the collection.

drop() removes the entire collection. remove() is deprecated. delete() is not a valid method.

❓ optimization

advanced

Performance difference between drop() and deleteMany({})

Which statement best describes the performance difference when removing all data from a large collection?

A Both have the same performance because they remove all data.

B drop() is faster because it removes the entire collection and indexes immediately, while deleteMany({}) deletes documents one by one.

C deleteMany({}) is faster because it uses bulk operations internally, drop() is slower due to index rebuilding.

D drop() is slower because it requires recreating the collection manually after.

💡 Hint

Think about what happens internally when a collection is dropped versus documents deleted.

drop() quickly removes the entire collection and all indexes, which is faster than deleting documents individually with deleteMany({}).

🔧 Debug

expert

Unexpected behavior after drop() and insert

After running db.orders.drop() and then inserting a document into orders, what is the state of the collection?

MongoDB

db.orders.drop()
db.orders.insertOne({item: 'book', qty: 3})
db.orders.getIndexes()

A The insertOne() fails because the collection was dropped.

B The collection does not exist because drop() permanently deletes it.

C The collection exists with the inserted document and all previous indexes restored.

D The collection exists with the inserted document but has only the default _id index.

💡 Hint

Consider what happens to indexes after dropping and recreating a collection.

After dropping a collection, all indexes are removed. Inserting a document recreates the collection with only the default _id index.

Practice

1. What does the drop() method do in MongoDB?

easy

A. Removes the entire collection and all its documents

B. Deletes only one document from the collection

C. Removes documents matching a filter but keeps the collection

D. Renames the collection

Solution

1. Step 1: Understand drop() purpose

   The drop() method deletes the whole collection including all documents inside it.

2. Step 2: Compare with other methods

   Unlike deleteMany(), which removes documents but keeps the collection, drop() removes the collection itself.

3. Final Answer:

   Removes the entire collection and all its documents -> Option A

4. Quick Check:

   drop() removes collection [OK]

Hint: Drop removes collection; deleteMany removes documents only [OK]

Common Mistakes:

• Thinking drop() only deletes documents, not the collection
• Confusing deleteMany() with drop()
• Assuming drop() keeps collection structure

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

easy

A. db.users.removeCollection()

B. db.users.drop()

C. db.users.deleteOne({})

D. db.users.deleteMany({})

Solution

1. Step 1: Identify method to delete all documents

   deleteMany({}) with an empty filter deletes all documents but keeps the collection.

2. Step 2: Check other options

   drop() deletes the whole collection, deleteOne({}) deletes one document, and removeCollection() is invalid syntax.

3. Final Answer:

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

4. Quick Check:

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

Hint: Use deleteMany({}) to clear all docs, not drop() [OK]

Common Mistakes:

• Using drop() which deletes the collection
• Using deleteOne() which deletes only one document
• Using non-existent removeCollection() method

3. Given a collection products with 100 documents, what will be the result after running db.products.deleteMany({ category: 'electronics' }) if 30 documents match the filter?

medium

A. All 100 documents are deleted and collection is dropped

B. 30 documents with category 'electronics' are deleted; 70 remain

C. No documents are deleted because filter is ignored

D. Collection is renamed to 'electronics'

Solution

1. Step 1: Understand deleteMany with filter

   The command deletes only documents matching the filter { category: 'electronics' }.

2. Step 2: Calculate remaining documents

   Since 30 documents match, they are deleted; 70 documents remain in the collection.

3. Final Answer:

   30 documents with category 'electronics' are deleted; 70 remain -> Option B

4. Quick Check:

   deleteMany(filter) deletes matching docs only [OK]

Hint: deleteMany(filter) deletes matching docs, not whole collection [OK]

Common Mistakes:

• Assuming deleteMany deletes entire collection
• Thinking filter is ignored
• Confusing deleteMany with drop

4. You want to remove all documents from the orders collection but accidentally run db.orders.dropMany({}). What is the issue?

medium

A. dropMany is not a valid MongoDB method

B. It deletes all documents but keeps the collection

C. It deletes only one document

D. It renames the collection

Solution

1. Step 1: Check method validity

   dropMany is not a valid MongoDB method; the correct methods are drop() and deleteMany().

2. Step 2: Understand consequences

   Since dropMany does not exist, this command will cause an error and no documents will be deleted.

3. Final Answer:

   dropMany is not a valid MongoDB method -> Option A

4. Quick Check:

   Invalid method causes error [OK]

Hint: Only drop() and deleteMany() exist; no dropMany() [OK]

Common Mistakes:

• Using dropMany() instead of deleteMany()
• Assuming dropMany deletes documents
• Confusing drop() and deleteMany() syntax

5. You want to completely remove a collection named logs only if it has fewer than 10 documents. Which sequence of commands correctly achieves this?

hard

A. Run db.logs.deleteMany({}) only

B. Run db.logs.deleteMany({}) then db.logs.drop()

C. Check count with db.logs.countDocuments(), then run db.logs.drop() if count < 10

D. Run db.logs.drop() without checking count

Solution

1. Step 1: Count documents in collection

   Use db.logs.countDocuments() to find how many documents exist.

2. Step 2: Conditionally drop collection

   If the count is less than 10, use db.logs.drop() to remove the entire collection.

3. Final Answer:

   Check count with db.logs.countDocuments(), then run db.logs.drop() if count < 10 -> Option C

4. Quick Check:

   Count first, then drop collection [OK]

Hint: Count documents first, then drop collection if condition met [OK]

Common Mistakes:

• Dropping collection without checking document count
• Deleting documents then dropping collection unnecessarily
• Using deleteMany() alone to remove collection