$limit and $skip stages in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this aggregation pipeline?

Given a collection products with documents:

{ "name": "Pen", "price": 5 }

{ "name": "Pencil", "price": 2 }

{ "name": "Eraser", "price": 1 }

{ "name": "Notebook", "price": 10 }

What will be the result of this pipeline?

[{ $skip: 1 }, { $limit: 2 }]

MongoDB

db.products.aggregate([{ $skip: 1 }, { $limit: 2 }])

A [{ "name": "Pencil", "price": 2 }, { "name": "Eraser", "price": 1 }]

B [{ "name": "Eraser", "price": 1 }, { "name": "Notebook", "price": 10 }]

C [{ "name": "Notebook", "price": 10 }]

D [{ "name": "Pen", "price": 5 }, { "name": "Pencil", "price": 2 }]

💡 Hint

Remember, $skip removes the first N documents, then $limit picks the next M documents.

The pipeline skips the first document (Pen), then limits the output to the next two documents (Pencil and Eraser).

🧠 Conceptual

intermediate

How do $limit and $skip affect the order of documents?

Consider a collection users with documents ordered by age ascending. If you apply [{ $skip: 3 }, { $limit: 2 }] without a $sort stage, what will be the output order?

A The documents will be sorted by age ascending automatically before skipping and limiting.

B The documents will be in the original collection order, skipping first 3 and returning next 2.

C The documents will be sorted by age descending automatically before skipping and limiting.

D The output will be empty because $sort is missing.

💡 Hint

Think about whether $skip and $limit change order by themselves.

$skip and $limit do not reorder documents. They only skip and limit documents in the current order.

📝 Syntax

advanced

Which pipeline is syntactically correct to skip 5 documents and then limit to 10?

Choose the correct MongoDB aggregation pipeline syntax:

A [{ skip: 5 }, { limit: 10 }]

B [{ $skip: '5' }, { $limit: '10' }]

C [{ $limit: 10 }, { $skip: 5 }]

D [{ $skip: 5 }, { $limit: 10 }]

💡 Hint

Check the stage operator names and value types.

The correct operators are $skip and $limit with numeric values. Order matters for expected results but syntax only requires valid operators.

❓ optimization

advanced

Which pipeline is more efficient to get the 10th to 20th documents sorted by date?

You want documents 10 to 20 sorted by date ascending. Which pipeline is better?

A [{ $sort: { date: 1 } }, { $skip: 9 }, { $limit: 11 }]

B [{ $skip: 9 }, { $limit: 11 }, { $sort: { date: 1 } }]

C [{ $limit: 20 }, { $skip: 9 }, { $sort: { date: 1 } }]

D [{ $sort: { date: 1 } }, { $limit: 11 }, { $skip: 9 }]

💡 Hint

Think about when sorting happens and how $skip and $limit reduce data.

Sorting first then skipping and limiting is efficient. Sorting after skipping/limiting can produce wrong results or be inefficient.

🔧 Debug

expert

Why does this pipeline return fewer documents than expected?

Pipeline:

[{ $limit: 5 }, { $skip: 3 }]

Expected 2 documents but got 0. Why?

A The pipeline returns 0 because $skip after $limit skips documents from an empty set.

B $skip runs first but is ignored because $limit is before it.

C $limit runs first and returns only 5 documents, then $skip removes 3 from those 5, leaving 2 documents.

D The pipeline returns 0 because $skip after $limit skips 3 documents from the 5 limited, but $skip cannot be after $limit.

💡 Hint

Remember the order stages run in the pipeline.

Stages run in order. $limit first returns 5 documents, then $skip removes 3 from those 5, leaving 2 documents output.