Why query patterns matter in MongoDB - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find all documents where the age is greater than 25.

MongoDB

db.users.find({"age": {"$gt": [1])

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

A20

B25

C30

D15

Attempts:

3 left

💡 Hint

Common Mistakes

Using a smaller number like 15 will include too many results.

Using $lt instead of $gt will find ages less than the number.

The $gt operator means 'greater than'. We want ages greater than 25, so the value should be 25.

2fill in blank

medium

Complete the code to find documents where the status is either 'active' or 'pending'.

MongoDB

db.users.find({"status": {"$in": [1])

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

A["active", "pending"]

B["active"]

C["inactive", "pending"]

D["pending", "deleted"]

Attempts:

3 left

💡 Hint

Common Mistakes

Using only one status will miss some results.

Using wrong statuses will return no results.

The $in operator checks if the field matches any value in the array. We want 'active' or 'pending', so the array should include both.

3fill in blank

hard

Fix the error in the query to find users with name starting with 'J'.

MongoDB

db.users.find({"name": [1])

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

A"^J"

B"J"

C/^J/

D/J$/

Attempts:

3 left

💡 Hint

Common Mistakes

Using a plain string 'J' matches exact name only.

Using /J$/ matches names ending with J, not starting.

To find names starting with 'J', use a regular expression /^J/. This matches strings beginning with 'J'.

4fill in blank

hard

Fill both blanks to find users older than 30 and with status 'active'.

MongoDB

db.users.find({"age": {"$gt": [1], "status": [2])

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

A30

B"active"

C"pending"

D25

Attempts:

3 left

💡 Hint

Common Mistakes

Using 25 instead of 30 for age.

Using 'pending' instead of 'active' for status.

The age should be greater than 30, so 30 is correct. The status should be exactly 'active', so use the string "active".

5fill in blank

hard

Fill all three blanks to find users with age less than 40, status 'pending', and name containing 'an'.

MongoDB

db.users.find({"age": {"[1]": [2], "status": "pending", "name": [3])

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

A$lt

B40

C/an/

D$gt

Attempts:

3 left

💡 Hint

Common Mistakes

Using $gt instead of $lt.

Using a string instead of regex for name.

Use $lt for 'less than' and 40 as the age limit. The name should match the regex /an/ to find 'an' anywhere in the name.

Practice

1. Why is it important to use good query patterns in MongoDB?

easy

A. They help find data faster and use less resources.

B. They make the database larger in size.

C. They slow down the application.

D. They remove data automatically.

Solution

1. Step 1: Understand query patterns

   Good query patterns are designed to find data quickly and efficiently.

2. Step 2: Recognize benefits

   Using good patterns reduces resource use and speeds up the app.

3. Final Answer:

   They help find data faster and use less resources. -> Option A

4. Quick Check:

   Good query patterns = faster data access [OK]

Hint: Good queries = faster results and less resource use [OK]

Common Mistakes:

• Thinking query patterns increase database size
• Believing query patterns slow down the app
• Assuming query patterns delete data

2. Which of the following is the correct way to write a simple MongoDB query to find documents where age is 25?

easy

A. db.collection.find({age: "25"})

B. db.collection.find(age == 25)

C. db.collection.find({age: 25})

D. db.collection.find({age: $eq: 25})

Solution

1. Step 1: Check MongoDB query syntax

   The correct syntax uses a JSON object with field and value inside find().

2. Step 2: Identify correct option

   db.collection.find({age: 25}) uses {age: 25} which is valid syntax for equality.

3. Final Answer:

   db.collection.find({age: 25}) -> Option C

4. Quick Check:

   Correct MongoDB query syntax = db.collection.find({age: 25}) [OK]

Hint: Use JSON object with field: value inside find() [OK]

Common Mistakes:

• Using == inside find() instead of JSON object
• Putting value as string when it should be number
• Incorrect use of $eq without braces

3. Given the collection users with documents:
{name: "Alice", age: 30}, {name: "Bob", age: 25}, {name: "Carol", age: 30}
What will db.users.find({age: 30}) return?

medium

A. [{name: "Bob", age: 25}]

B. [{name: "Alice", age: 30}, {name: "Carol", age: 30}]

C. []

D. SyntaxError

Solution

1. Step 1: Understand the query condition

   The query looks for documents where age equals 30.

2. Step 2: Match documents in collection

   Alice and Carol have age 30, so both match and are returned.

3. Final Answer:

   [{name: "Alice", age: 30}, {name: "Carol", age: 30}] -> Option B

4. Quick Check:

   Find age 30 returns Alice and Carol [OK]

Hint: Match field value exactly to get correct documents [OK]

Common Mistakes:

• Returning documents with age 25
• Expecting empty result
• Thinking query causes syntax error

4. What is wrong with this MongoDB query to find users older than 20?
db.users.find({age: > 20})

medium

A. The field name 'age' is misspelled.

B. The find() method cannot use comparison operators.

C. The query is missing a closing parenthesis.

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

Solution

1. Step 1: Analyze query syntax

   The query uses > directly inside the object, which is invalid in MongoDB.

2. Step 2: Correct usage of comparison operators

   MongoDB requires operators like $gt inside an object: {age: {$gt: 20}}.

3. Final Answer:

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

4. Quick Check:

   Use $gt for greater than in queries [OK]

Hint: Use $gt, $lt for comparisons inside query objects [OK]

Common Mistakes:

• Using > directly instead of $gt
• Assuming find() can't use comparisons
• Ignoring correct syntax for operators

5. You want to speed up queries that search users by their email. Which query pattern helps most in MongoDB?

hard

A. Use a simple equality query on the email field and create an index on email.

B. Use a regex query on email without any index.

C. Scan all documents and filter emails in application code.

D. Store emails in a separate collection without indexes.

Solution

1. Step 1: Identify efficient query pattern

   Simple equality queries on indexed fields are fastest in MongoDB.

2. Step 2: Understand index role

   Creating an index on email lets MongoDB find matches quickly without scanning all documents.

3. Final Answer:

   Use a simple equality query on the email field and create an index on email. -> Option A

4. Quick Check:

   Index + simple query = fast search [OK]

Hint: Index fields you query often for speed [OK]

Common Mistakes:

• Using regex without index causes slow queries
• Filtering in app wastes resources
• Not indexing searchable fields