MongoDB Concepts 1

Which document formats are typically used for documents stored in a document-based NoSQL database?

• XML and CSV format.

• XML and JSON format.

• CSV and JSON format.

• PDF and JSON format.

Answer explanation

Document-based NoSQL databases commonly use XML and JSON formats for storing data due to their flexibility and ability to represent complex structures. Thus, the correct answer is XML and JSON format.

Which of the following tasks cannot be completed with an aggregation pipeline? (Select one.)

• Filtering for relevant pieces of data

• Finding data from outside sources

• Grouping documents

• Calculating total values from a field across many documents

Answer explanation

The aggregation pipeline can filter, group, and calculate values from documents within the database. However, it cannot access or find data from outside sources, making 'Finding data from outside sources' the correct choice.

You have a collection that contains zip codes in the United States. Here’s a sample document from this collection:

{ "_id": ObjectId('5c8eccc1caa187d17ca6eea2'),

"city": "EVERGREEN",

"zip": "36401",

"loc": { "y": 31.458009, "x": 86.925771 },

"pop": 8556, "state": "AL" }

What will the output of this aggregation pipeline be? (Select one.)

db.zips.aggregate([ { $match: { "state": "CA" } }, { $group: { "_id": "$zip" } } ])

• One document for each city located in California (CA)

• One document for each zip code located in California (CA)

• The total number of documents that contain a zip code in the state of California (CA)

• All documents that contain a zip where the state is California (CA)

Answer explanation

The aggregation pipeline first filters documents to only those with state 'CA'. Then, it groups by zip code, resulting in one document for each unique zip code in California.

What does the $count stage return? (Select one.)

• A single document with one field that contains the value set to the number of documents at this stage in the aggregation pipeline.

• The number of groups in an aggregation pipeline.

• A set number of documents from the aggregation pipeline.

• A single document that contains the number of fields that have been modified in an aggregation pipeline.

Answer explanation

The $count stage returns a single document with one field that indicates the total number of documents processed at that point in the aggregation pipeline, making the first choice correct.

2. Which query operator in MongoDB is used to compare if a value is greater than another?

• • $lt

• $gte

• $gt

• $eq

Answer explanation

In MongoDB, the $gt operator is used to compare if a value is greater than another. The other options serve different purposes: $lt checks for less than, $gte for greater than or equal to, and $eq for equality.

3. Which of the following is not a valid stage in an aggregation pipeline in MongoDB?

• $match

• $sort

• $join

• $group

Answer explanation

In MongoDB's aggregation pipeline, valid stages include $match, $sort, and $group. However, $join is not a valid stage; instead, MongoDB uses $lookup for joining collections.

4. Given the following document in a products collection:

{ "_id": ObjectId('5c8eccc1caa187d17ca6eea2'),

"name": "Laptop",

"price": 1200,

"category": "Electronics"}

What does the query db.products.find({ price: { $lt: 1500 } }) return?

• • All documents with a price less than 1500.

• • All documents with a price greater than 1500.

• • All documents with a price equal to 1500.

• • No documents, since $lt is not a valid operator.

Answer explanation

The query db.products.find({ price: { $lt: 1500 } }) returns all documents with a price less than 1500. Since the document has a price of 1200, it meets this condition.

5. What is the primary purpose of the $group stage in an aggregation pipeline?

• • To sort documents in ascending or descending order.

• • To filter documents based on a specific criterion.

• • To group documents by a specific field and perform aggregate calculations.

• • To limit the number of documents returned.

Answer explanation

The $group stage in an aggregation pipeline is primarily used to group documents by a specific field and perform aggregate calculations, such as summing or averaging values, making it essential for data analysis.

6. What is the output of the following aggregation pipeline?

db.orders.aggregate([

{ $match: { status: "completed" } },

{ $group: { _id: "$customerId", totalSpent: { $sum: "$amount" } } } ])

• • The total sum of all completed orders.

• • One document per customer with the total spent on completed orders.

• • All completed orders without modifications.

• • A single document with the total number of completed orders.

Answer explanation

The aggregation pipeline first filters for completed orders, then groups them by customerId, summing the amounts. This results in one document per customer showing the total spent on completed orders.

7. Which of the following aggregation stages is used to filter documents before grouping them?

• • $project

• $match

• $group

• $sort

Answer explanation

The $match stage is used to filter documents before they are grouped. It allows you to specify criteria to limit the documents that will be processed in subsequent stages, such as $group.