Delete with filter conditions in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Delete documents with a specific field value

Given a collection users with documents containing a field status, what will be the result of this delete operation?

MongoDB

db.users.deleteMany({ status: "inactive" })

A Throws an error because the filter condition is invalid.

B Deletes only one document where the status is "inactive" and returns the deleted document.

C Deletes all documents where the status is exactly "inactive" and returns the count of deleted documents.

D Deletes all documents regardless of status and returns the count of deleted documents.

💡 Hint

Remember that deleteMany removes all documents matching the filter.

The deleteMany method deletes all documents matching the filter condition. Here, it deletes all users with status equal to "inactive" and returns the count of deleted documents.

❓ query_result

intermediate

Delete documents with multiple filter conditions

What documents will be deleted by this command?

MongoDB

db.orders.deleteMany({ status: "pending", amount: { $lt: 100 } })

A Deletes all orders with status "pending" and amount less than 100.

B Deletes all orders with status "pending" or amount less than 100.

C Deletes only one order with status "pending" and amount less than 100.

D Deletes all orders regardless of status or amount.

💡 Hint

Multiple conditions inside the filter object are combined with AND logic.

The filter specifies two conditions combined with AND: status must be "pending" AND amount must be less than 100. So only documents matching both are deleted.

📝 Syntax

advanced

Identify the syntax error in delete command

Which option contains a syntax error in the MongoDB delete command?

A db.products.deleteMany({ price: { $gt: 50 } })

B db.products.deleteMany({ price: $gt: 50 })

C db.products.deleteOne({ category: "electronics" })

D db.products.deleteMany({ stock: 0 })

💡 Hint

Check the correct way to write filter conditions with operators.

Option B is invalid because the operator $gt must be inside an object: { price: { $gt: 50 } }. Option B is correct.

❓ optimization

advanced

Optimize deletion of large number of documents

You want to delete millions of documents matching a filter. Which approach is best to avoid performance issues?

A Use <code>deleteMany</code> with the filter directly in one command.

B Use <code>deleteOne</code> repeatedly until all documents are deleted.

C Drop the entire collection and recreate it.

D Delete documents in batches using a loop with <code>deleteMany</code> and a limit on each batch.

💡 Hint

Deleting huge amounts at once can cause locks or timeouts.

Deleting millions of documents at once can cause performance problems. Deleting in batches with limits reduces load and avoids long locks.

🧠 Conceptual

expert

Effect of delete with empty filter

What happens if you run db.collection.deleteMany({})?

MongoDB

db.collection.deleteMany({})

A Deletes all documents in the collection.

B Deletes no documents because the filter is empty.

C Throws an error because the filter cannot be empty.

D Deletes only documents with null fields.

💡 Hint

An empty filter matches all documents.

An empty filter {} matches every document in the collection, so deleteMany({}) deletes all documents.

Practice

1. What does the deleteMany method do in MongoDB when used with a filter condition?

easy

A. Deletes the entire collection regardless of the filter.

B. Deletes only the first document in the collection.

C. Deletes all documents that match the filter condition.

D. Updates documents instead of deleting them.

Solution

1. Step 1: Understand deleteMany purpose

   deleteMany is designed to remove multiple documents matching a filter.

2. Step 2: Apply filter condition effect

   Only documents matching the filter are deleted, not the entire collection.

3. Final Answer:

   Deletes all documents that match the filter condition. -> Option C

4. Quick Check:

   deleteMany removes all matching docs [OK]

Hint: Remember: deleteMany removes all matching documents [OK]

Common Mistakes:

• Confusing deleteMany with deleteOne
• Thinking deleteMany deletes entire collection
• Assuming deleteMany updates documents

2. Which of the following is the correct syntax to delete one document where the field status equals "inactive" in MongoDB?

easy

A. db.collection.deleteOne({status: "inactive"})

B. db.collection.delete({status: "inactive"})

C. db.collection.removeOne({status: "inactive"})

D. db.collection.deleteMany({status == "inactive"})

Solution

1. Step 1: Identify correct method for deleting one document

   The method to delete a single document is deleteOne.

2. Step 2: Check filter syntax correctness

   The filter uses a key-value pair with colon, not double equals or other syntax.

3. Final Answer:

   db.collection.deleteOne({status: "inactive"}) -> Option A

4. Quick Check:

   Correct method and filter syntax = db.collection.deleteOne({status: "inactive"}) [OK]

Hint: Use deleteOne with colon syntax for filters [OK]

Common Mistakes:

• Using delete instead of deleteOne
• Using == instead of : in filter
• Using removeOne which does not exist

3. Given the collection users with documents:
{name: "Alice", age: 30}
{name: "Bob", age: 25}
{name: "Charlie", age: 30}
What will be the result after running db.users.deleteMany({age: 30})?

medium

A. Only Alice's document is deleted.

B. Both Alice's and Charlie's documents are deleted.

C. Only Bob's document is deleted.

D. No documents are deleted.

Solution

1. Step 1: Identify filter condition effect

   The filter {age: 30} matches documents where age is exactly 30.

2. Step 2: Determine matching documents

   Alice and Charlie both have age 30, so both match and will be deleted by deleteMany.

3. Final Answer:

   Both Alice's and Charlie's documents are deleted. -> Option B

4. Quick Check:

   deleteMany removes all matching docs = Both Alice's and Charlie's documents are deleted. [OK]

Hint: deleteMany removes all docs matching filter, not just one [OK]

Common Mistakes:

• Deleting only one document with deleteMany
• Deleting documents not matching filter
• Confusing age 25 with 30

4. You run the command db.orders.deleteOne({orderId: 12345}) but no documents are deleted. What could be the problem?

medium

A. MongoDB does not support deleteOne method.

B. deleteOne deletes all documents, so it should have deleted more.

C. You must use deleteMany to delete any documents.

D. The filter field name or value might be incorrect or missing in documents.

Solution

1. Step 1: Understand deleteOne behavior

   deleteOne deletes only one document matching the filter if it exists.

2. Step 2: Check filter correctness

   If no document matches {orderId: 12345}, nothing is deleted. The field or value may be wrong or missing.

3. Final Answer:

   The filter field name or value might be incorrect or missing in documents. -> Option D

4. Quick Check:

   No matching document means no deletion [OK]

Hint: Check filter matches existing documents before deleting [OK]

Common Mistakes:

• Assuming deleteOne deletes all documents
• Using deleteMany when deleteOne is intended
• Thinking deleteOne does not exist

5. You want to delete all documents from the products collection where the stock field is less than or equal to 0. Which MongoDB command correctly achieves this?

hard

A. db.products.deleteMany({stock: {$lte: 0}})

B. db.products.deleteOne({stock <= 0})

C. db.products.remove({stock: {$lt: 0}})

D. db.products.deleteMany({stock: {$gte: 0}})

Solution

1. Step 1: Identify correct operator for less than or equal

   The MongoDB operator for less than or equal is $lte.

2. Step 2: Choose correct method and filter syntax

   deleteMany deletes all matching documents; filter must use {stock: {$lte: 0}}.

3. Final Answer:

   db.products.deleteMany({stock: {$lte: 0}}) -> Option A

4. Quick Check:

   Use $lte with deleteMany for all matching docs [OK]

Hint: Use $lte operator inside deleteMany filter for <= condition [OK]

Common Mistakes:

• Using incorrect comparison syntax like stock <= 0
• Using deleteOne instead of deleteMany for multiple docs
• Using $gte instead of $lte