Delete with filter conditions in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the deleteOne() method do in MongoDB?

It deletes the first document that matches the filter condition in a collection.

beginner

How do you delete multiple documents matching a condition in MongoDB?

Use the deleteMany() method with a filter to remove all documents that match the condition.

intermediate

What happens if you call deleteOne({}) with an empty filter?

It deletes the first document in the collection because the filter matches all documents.

beginner

Explain the filter condition in MongoDB delete operations.

The filter is a query object that specifies which documents to delete based on field values.

beginner

What is the difference between deleteOne() and deleteMany()?

deleteOne() removes only the first matching document, while deleteMany() removes all matching documents.

Which MongoDB method deletes all documents matching a filter?

A deleteMany()

B removeMany()

C removeOne()

D deleteOne()

deleteMany() deletes all documents that match the filter condition.

What does db.collection.deleteOne({ age: 30 }) do?

A Deletes documents where age is not 30

B Deletes all documents where age is 30

C Deletes the first document where age is 30

D Deletes the entire collection

It deletes only the first document matching the filter { age: 30 }.

If you want to delete documents where status is 'inactive', which filter is correct?

A { status: 'inactive' }

B { status: 'active' }

C { status: { $ne: 'inactive' } }

D {}

The filter { status: 'inactive' } matches documents with status equal to 'inactive'.

What happens if you call deleteMany({})?

A Deletes no documents

B Deletes all documents in the collection

C Deletes the first document only

D Throws an error

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

Which method would you use to delete a single document by its unique _id?

A removeMany({ _id: id })

B deleteMany({ _id: id })

C remove({ _id: id })

D deleteOne({ _id: id })

deleteOne() is used to delete a single document matching the filter.

Describe how to delete documents in MongoDB using filter conditions.

Think about how you tell MongoDB which documents to remove.

What is the difference between deleteOne() and deleteMany() in MongoDB?

Focus on how many documents each method removes.

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