Why querying is essential in MongoDB - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find all documents in the collection.

MongoDB

db.collection.find([1])

Drag options to blanks, or click blank then click option'

A{}

B[]

Cnull

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Using an empty array instead of an empty object.

Using null which does not work as a query filter.

Using an empty object {} as the query returns all documents in the collection.

2fill in blank

medium

Complete the code to find documents where age is 25.

MongoDB

db.collection.find({ age: [1] })

Drag options to blanks, or click blank then click option'

Atrue

B"25"

C{ $eq: 25 }

D25

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around 25, which makes it a string.

Using a query operator without the correct syntax.

Use the number 25 without quotes to match the age field exactly.

3fill in blank

hard

Fix the error in the query to find documents with name 'Alice'.

MongoDB

db.collection.find({ name: [1] })

Drag options to blanks, or click blank then click option'

AAlice

B"Alice"

C{ $eq: Alice }

D'Alice'

Attempts:

3 left

💡 Hint

Common Mistakes

Not using quotes around string values.

Using single quotes which may cause errors in some environments.

String values in MongoDB queries must be in double quotes.

4fill in blank

hard

Fill both blanks to find documents where age is greater than 30.

MongoDB

db.collection.find({ age: { [1]: [2] } })

Drag options to blanks, or click blank then click option'

A$gt

B30

C25

D$lt

Attempts:

3 left

💡 Hint

Common Mistakes

Using $lt instead of $gt.

Using a wrong number value.

The operator $gt means 'greater than', so { age: { $gt: 30 } } finds ages over 30.

5fill in blank

hard

Fill all three blanks to find documents where name starts with 'J' and age is less than 40.

MongoDB

db.collection.find({ name: { [1]: [2] }, age: { [3]: 40 } })

Drag options to blanks, or click blank then click option'

A$regex

B"^J"

C$lt

D$gt

Attempts:

3 left

💡 Hint

Common Mistakes

Using $gt instead of $lt for age.

Not using regex correctly for name.

Use $regex with "^J" to match names starting with 'J', and $lt to find ages less than 40.

Practice

1. Why is querying essential when working with a MongoDB database?

easy

A. It automatically fixes errors in the database.

B. It deletes all data from the database.

C. It helps find specific data quickly and efficiently.

D. It creates new databases without user input.

Solution

1. Step 1: Understand the purpose of querying

   Querying is used to search and retrieve specific data from a database.

2. Step 2: Identify the correct benefit of querying

   Querying helps users find data quickly and efficiently, not to delete or fix data automatically.

3. Final Answer:

   It helps find specific data quickly and efficiently. -> Option C

4. Quick Check:

   Querying = Find data fast [OK]

Hint: Queries help you get only the data you want fast [OK]

Common Mistakes:

• Thinking querying fixes database errors automatically
• Confusing querying with deleting data
• Believing querying creates databases

2. Which of the following is the correct MongoDB query syntax to find all documents in a collection named users?

easy

A. db.users.find()

B. db.users.get()

C. db.users.search()

D. db.users.select()

Solution

1. Step 1: Recall MongoDB query syntax for fetching documents

   The correct method to retrieve documents is find().

2. Step 2: Check each option's method name

   Only find() is valid; others like get(), search(), and select() are invalid in MongoDB.

3. Final Answer:

   db.users.find() -> Option A

4. Quick Check:

   Find method = db.collection.find() [OK]

Hint: Use .find() to get documents from a collection [OK]

Common Mistakes:

• Using .get() or .search() instead of .find()
• Forgetting parentheses after find
• Confusing SQL syntax with MongoDB

3. Given the collection products with documents:

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

{"name": "Notebook", "price": 15}

{"name": "Eraser", "price": 3}

What will the query db.products.find({ price: { $gt: 4 } }) return?

medium

A. []

B. [{"name": "Eraser", "price": 3}]

C. [{"name": "Notebook", "price": 15}]

D. [{"name": "Pen", "price": 5}, {"name": "Notebook", "price": 15}]

Solution

1. Step 1: Understand the query filter

   The query filters documents where price is greater than 4.

2. Step 2: Check each document's price

   Pen (5) and Notebook (15) have prices > 4; Eraser (3) does not.

3. Final Answer:

   [{"name": "Pen", "price": 5}, {"name": "Notebook", "price": 15}] -> Option D

4. Quick Check:

   Price > 4 returns Pen and Notebook [OK]

Hint: Look for documents matching the filter condition carefully [OK]

Common Mistakes:

• Including documents with price equal to 4
• Selecting documents with price less than 4
• Misunderstanding $gt operator

4. What is wrong with this MongoDB query to find users older than 25?
db.users.find({ age: > 25 })

medium

A. The query is missing a closing parenthesis.

B. The comparison operator > is used incorrectly inside the query object.

C. The find() method cannot filter by age.

D. The collection name should be in quotes.

Solution

1. Step 1: Analyze the query syntax

   The query uses age: > 25, which is invalid syntax in MongoDB.

2. Step 2: Identify correct operator usage

   MongoDB requires operators like $gt inside an object: { age: { $gt: 25 } }.

3. Final Answer:

   The comparison operator > is used incorrectly inside the query object. -> Option B

4. Quick Check:

   Use $gt for greater than in queries [OK]

Hint: Use $gt, not >, inside query objects [OK]

Common Mistakes:

• Using > directly instead of $gt
• Putting collection name in quotes unnecessarily
• Assuming find() can't filter by fields

5. You have a collection orders with documents containing status and total. How would you write a query to find all orders with status "shipped" and total greater than 100?

hard

A. db.orders.find({ status: "shipped", total: { $gt: 100 } })

B. db.orders.find({ status: { $eq: "shipped" }, total: > 100 })

C. db.orders.find({ status == "shipped" && total > 100 })

D. db.orders.find({ status: "shipped" || total: { $gt: 100 } })

Solution

1. Step 1: Understand the query conditions

   We want orders where status equals "shipped" AND total is greater than 100.

2. Step 2: Check correct MongoDB syntax for AND conditions

   MongoDB treats multiple fields in the query object as AND conditions. Use { status: "shipped", total: { $gt: 100 } }.

3. Final Answer:

   db.orders.find({ status: "shipped", total: { $gt: 100 } }) -> Option A

4. Quick Check:

   Multiple fields = AND in MongoDB queries [OK]

Hint: Use multiple fields in find() for AND conditions [OK]

Common Mistakes:

• Using > instead of $gt inside query
• Using == or && which are invalid in MongoDB queries
• Using || which means OR, not AND