Query patterns that cause collection scans in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find documents where the field 'age' is exactly 30.

MongoDB

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

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

A{ $eq: 30 }

B"30"

C{ age: 30 }

D30

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string '30' instead of number 30 causes no match if the field is numeric.

Using a nested object incorrectly inside the value.

The query looks for documents where the 'age' field equals the number 30. Using 30 directly matches the numeric value.

2fill in blank

medium

Complete the code to find documents where the 'name' field starts with 'A'.

MongoDB

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

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

A/^A/

B"A%"

C{ $regex: 'A' }

D{ $startsWith: 'A' }

Attempts:

3 left

💡 Hint

Common Mistakes

Using SQL-like wildcards such as 'A%' which MongoDB does not understand.

Using incorrect operators like $startsWith which do not exist.

The regular expression /^A/ matches strings starting with 'A'. This is the correct way to query for names starting with 'A' in MongoDB.

3fill in blank

hard

Fix the error in the query to find documents where 'score' is greater than 50.

MongoDB

db.scores.find({"score": { [1] 50 } })

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

A$lt:

B$gte:

C$gt:

D$eq:

Attempts:

3 left

💡 Hint

Common Mistakes

Using $gte includes equal values, which is not the same as strictly greater than.

Using $lt or $eq will not find values greater than 50.

The operator $gt means 'greater than'. It must be used as a key in an object to compare values.

4fill in blank

hard

Fill both blanks to create a query that finds documents where 'status' is 'active' and 'age' is less than 40.

MongoDB

db.users.find({"status": [1], "age": { [2] 40 } })

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

A"active"

B$lt:

C$gt:

D"inactive"

Attempts:

3 left

💡 Hint

Common Mistakes

Using $gt instead of $lt for age.

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

The 'status' field should match the string 'active'. The 'age' field uses the $lt operator to find values less than 40.

5fill in blank

hard

Fill all three blanks to create a query that finds documents where the 'category' is 'books', 'price' is greater than 20, and 'inStock' is true.

MongoDB

db.products.find({"category": [1], "price": { [2] 20 }, "inStock": [3])

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

A"books"

B$gt:

Ctrue

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using false instead of true for inStock.

Using $lt instead of $gt for price.

The 'category' field matches the string 'books'. The 'price' field uses $gt to find prices greater than 20. The 'inStock' field is a boolean true.