C100DBA CRUD Operations Practice Test 2 — 15 Questions

Question 1

When using the find() method, which cursor modifier is used to skip a specific number of documents before returning results?

Accepted Answer

.skip(). The skip() method is the standard cursor modifier used to bypass a set number of initial documents in a result set. It is essential for implementing pagination in applications, often used in conjunction with limit() to fetch specific pages of data. Understanding its impact on performance is vital, as skipping large numbers of documents can be expensive for the database engine to process.

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.offset()

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.limit()

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.next()

Question 2

Refer to the exhibit. Which documents will this query return?

Accepted Answer

The 5 cheapest products tagged 'electronics' that cost less than 500.. This query retrieves documents containing 'electronics' in the 'tags' array where the price is less than 500. The results are ordered by price in ascending order, and only the first 5 documents are returned. This combination demonstrates how to perform complex filtering, sorting, and limiting, which is fundamental for building performant search and discovery features in MongoDB-backed applications.

Answer

All documents with 'electronics' tag sorted by price ascending.

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Any 5 products with price less than 500, regardless of tags.

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The 5 most expensive products tagged 'electronics' that cost less than 500.

Question 3

Which operator would you use to remove an element from an array field in a document?

Accepted Answer

$pull. The $pull operator is the primary tool for removing items from arrays based on a specified query condition. It is highly effective because it allows you to remove one or many matching elements in a single operation without needing to pull the entire document into application memory, modify the array, and push it back. This promotes efficient resource usage and data consistency.

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$remove

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$pop

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$unset

Question 4

What happens when you perform a find() query on a collection where the projection includes both an inclusion and an exclusion for different fields?

Accepted Answer

The query returns an error.. MongoDB enforces strict rules regarding field projection. You cannot mix inclusions and exclusions in the same projection object, with the exception of the _id field. This design choice ensures predictable behavior for the result set. Understanding this limitation is important for developers when they need to fetch specific subsets of data while omitting others, as it prevents common errors during query construction.

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The inclusion takes precedence over the exclusion.

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The exclusion takes precedence over the inclusion.

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It returns the intersection of both fields.

Question 5

What is the primary function of the 'projection' parameter in the find() method?

Accepted Answer

To specify which fields to return in the documents.. Projections allow you to limit the data returned by a query, which is a best practice for reducing network traffic and memory consumption on both the client and the server. By requesting only the fields required for the current task, you optimize application performance and security. This is a foundational concept in database interaction that helps maintain efficient data flow in large-scale applications.

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To filter which documents are returned.

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To sort the returned documents.

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To limit the number of documents returned.

Question 6

Refer to the exhibit. What is the effect of the 'writeConcern' parameter in this operation?

Accepted Answer

It ensures the data is replicated to most nodes before acknowledgment.. The write concern {w: 'majority'} ensures that the operation is only acknowledged after it has been written to a majority of the replica set members. This is essential for data durability and preventing data loss in the event of a primary node failover. It provides a stronger guarantee than the default 'w: 1' by ensuring that data is replicated before the application proceeds.

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It forces the update to occur only on the primary node.

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It only updates documents that have a majority of fields set.

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It makes the update operation read-only.

Question 7

Which method is the safest way to delete a single document from a collection based on a unique identifier?

Accepted Answer

db.collection.deleteOne(filter). The deleteOne method is the safest way to remove a single document because it accepts a filter and stops after deleting the first matching document, even if multiple documents match. This prevents accidental data loss that could occur with deleteMany if the filter criteria were not sufficiently specific. It is a fundamental practice to use the most restrictive method possible for destructive operations.

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db.collection.remove(filter)

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db.collection.deleteMany(filter)

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db.collection.drop()

Question 8

What happens if you run an update operation with the $set operator on a field that does not yet exist in the document?

Accepted Answer

The field is created with the new value.. MongoDB is schema-flexible, meaning it will create the field if it is missing during an update operation. The $set operator is intelligent enough to add the field with the specified value, which simplifies code by eliminating the need for pre-checks or initialization logic. This is a core strength of the document model that allows for evolving data requirements without requiring explicit database migrations.

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The update fails with an error.

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The update is ignored.

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The entire document is replaced.

Question 9

Which THREE of the following are valid MongoDB CRUD query operators?

Accepted Answer

$gt. MongoDB provides a rich set of query operators for filtering and modifying data. Operators like $gt, $ne, and $exists are staples of the query language. Mastering these allows developers to construct complex, powerful queries to extract specific data subsets. Understanding the syntax and behavior of these operators is a key requirement for the C100DBA certification and effective day-to-day database administration work.

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$and_not

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$is_null

Question 10

Which cursor method should you use to sort documents by a field named 'age' in descending order?

Accepted Answer

.sort({age: -1}). The sort() method is used to define the order of results returned by a query. Passing {age: -1} ensures that the documents are ordered from oldest to youngest. Mastering sorting is essential for returning data in a meaningful way to users, such as showing the most recent posts or the highest-rated products first, which are common requirements for almost all web and mobile applications.

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.sort({age: 1})

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.orderBy({age: 'desc'})

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.order({age: 'desc'})

Question 11

Which method should you use to update multiple documents in a collection and insert a new document if no matches are found?

Accepted Answer

db.collection.updateMany({filter}, {update}, {upsert: true}). The updateMany method modifies all documents matching the filter. When combined with the 'upsert' option set to true, it ensures that if no documents match the query criteria, a single new document is created containing the filter and update criteria. This pattern is critical for idempotent data ingestion pipelines where you must guarantee existence of records without performing separate check-then-insert logic, thus preventing race conditions.

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db.collection.updateOne()

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db.collection.replaceOne()

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db.collection.save()

Question 12

A developer needs to retrieve a subset of fields from a collection. Which TWO methods achieve this using projection?

Accepted Answer

db.collection.find(query, projection). Projection allows developers to minimize network bandwidth and memory usage by returning only necessary fields from documents. Both find() and findOne() support a second argument that defines the projection document. This is a fundamental optimization technique in MongoDB to prevent the 'select *' anti-pattern, ensuring that large fields like binary data or embedded arrays are not fetched when they are not required by the application layer.

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db.collection.aggregate(pipeline, projection)

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db.collection.findAndModify(query, projection)

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db.collection.update(query, projection)

Question 13

Which operator is best used to add an element to an array only if it does not already exist?

Accepted Answer

$addToSet. The $addToSet operator is the standard MongoDB tool for array manipulation when uniqueness is required. It ensures that the specified value is added to the array only if it is not currently present. This prevents duplicate entries without requiring the application to perform a read-then-write check, making it an atomic and highly performant operation for managing sets within document-based data structures.

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$push

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$set

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$each

Question 14

Which update operator would you use to increase the value of a numeric field by a specific amount?

Accepted Answer

$inc. The $inc operator is the standard tool for atomic numeric increments. It is highly efficient because it allows the database to perform the math on the server side without requiring the client to read the current value, perform the addition locally, and write it back. This eliminates race conditions where two clients might read the same value and overwrite each other's increments.

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$add

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$set

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$mul

Question 15

Refer to the exhibit. What is the impact of this operation on the document?

Accepted Answer

It deletes the 'qty' field and creates a new 'quantity' field with the same value.. The $rename operator updates the name of a field within a document. It is atomic at the document level. If the field 'qty' exists, it is renamed to 'quantity'. If 'qty' does not exist, the operation does nothing. This is useful for schema migrations or refactoring data models without needing to delete and re-insert the document, which preserves the original _id and other field values.

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It fails because 'qty' is not a field in the document.

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It replaces the entire document with a new one containing only the quantity field.

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It creates a new collection named 'quantity'.