$lt and $lte for less than in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents with age less than 30

Given a collection users with documents containing an age field, which query returns all users younger than 30?

MongoDB

db.users.find({ age: { $lt: 30 } })

A db.users.find({ age: { $gt: 30 } })

B db.users.find({ age: { $lte: 30 } })

C db.users.find({ age: { $lt: 30 } })

D db.users.find({ age: { $gte: 30 } })

💡 Hint

Remember, $lt means strictly less than, while $lte means less than or equal to.

The $lt operator selects documents where the value is strictly less than the specified number. Here, it returns users with age less than 30.

❓ query_result

intermediate

Find documents with price less than or equal to 100

Which query returns all products with a price less than or equal to 100?

MongoDB

db.products.find({ price: { $lte: 100 } })

A db.products.find({ price: { $lte: 100 } })

B db.products.find({ price: { $lt: 100 } })

C db.products.find({ price: { $gt: 100 } })

D db.products.find({ price: { $gte: 100 } })

💡 Hint

Check if the query includes products priced exactly at 100.

The $lte operator includes values less than or equal to the specified number, so products priced at 100 are included.

📝 Syntax

advanced

Identify the syntax error in this query

Which option contains a syntax error when trying to find documents with score less than 50?

MongoDB

db.scores.find({ score: { $lt 50 } })

A db.scores.find({ score: { $lt: '50' } })

B db.scores.find({ score: { $lt: 50 } })

C db.scores.find({ score: { $lte: 50 } })

D db.scores.find({ score: { $lt 50 } })

💡 Hint

Check the syntax for specifying operators and values in MongoDB queries.

Option D is missing the colon : between $lt and the value, causing a syntax error.

❓ query_result

advanced

Find documents with date less than or equal to a specific date

Given documents with a date field, which query returns all documents with date less than or equal to January 1, 2023?

MongoDB

db.events.find({ date: { $lte: new Date('2023-01-01') } })

A db.events.find({ date: { $lt: new Date('2023-01-01') } })

B db.events.find({ date: { $lte: new Date('2023-01-01') } })

C db.events.find({ date: { $gt: new Date('2023-01-01') } })

D db.events.find({ date: { $gte: new Date('2023-01-01') } })

💡 Hint

Remember that $lte includes the date itself.

The $lte operator includes documents where the date is exactly January 1, 2023, or earlier.

🧠 Conceptual

expert

Understanding difference between $lt and $lte in query results

If a collection items has documents with a numeric field value containing values 10, 20, 30, and 40, what is the number of documents returned by db.items.find({ value: { $lt: 30 } }) compared to db.items.find({ value: { $lte: 30 } })?

A The first query returns 2 documents; the second returns 3 documents

B Both queries return 2 documents

C The first query returns 3 documents; the second returns 2 documents

D Both queries return 3 documents

💡 Hint

Think about whether the value 30 is included in each query.

The $lt query returns documents with values less than 30 (10 and 20), so 2 documents. The $lte query includes 30 as well, so it returns 3 documents.

Practice

1. What does the MongoDB operator $lt do in a query?

easy

A. Finds documents where the field value is less than a specified value

B. Finds documents where the field value is greater than a specified value

C. Finds documents where the field value is equal to a specified value

D. Finds documents where the field value is not equal to a specified value

Solution

1. Step 1: Understand the meaning of $lt

   The $lt operator means "less than" in MongoDB queries.

2. Step 2: Apply the operator meaning to query behavior

   It selects documents where the field value is strictly less than the given value.

3. Final Answer:

   Finds documents where the field value is less than a specified value -> Option A

4. Quick Check:

   $lt means less than [OK]

Hint: Remember: lt = less than, lte = less than or equal [OK]

Common Mistakes:

• Confusing $lt with $lte
• Thinking $lt means less than or equal
• Mixing $lt with greater than operators

2. Which of the following is the correct MongoDB query syntax to find documents where the field age is less than or equal to 30?

easy

A. { age: { $lte: 30 } }

B. { age: { $lt: 30 } }

C. { age: { $gt: 30 } }

D. { age: { $gte: 30 } }

Solution

1. Step 1: Identify the operator for less than or equal

   The operator $lte means "less than or equal to" in MongoDB.

2. Step 2: Match the operator with the query syntax

   The correct syntax to find documents where age is less than or equal to 30 is { age: { $lte: 30 } }.

3. Final Answer:

   { age: { $lte: 30 } } -> Option A

4. Quick Check:

   $lte means less than or equal [OK]

Hint: Use $lte for less than or equal, $lt for strictly less [OK]

Common Mistakes:

• Using $lt instead of $lte for less than or equal
• Using greater than operators by mistake
• Incorrect query object structure

3. Given the collection products with documents:

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

{ "name": "Notebook", "price": 10 }

{ "name": "Bag", "price": 20 }

What will be the result of the query db.products.find({ price: { $lt: 10 } })?

medium

A. []

B. [{ "name": "Pen", "price": 5 }, { "name": "Notebook", "price": 10 }]

C. [{ "name": "Pen", "price": 5 }]

D. [{ "name": "Notebook", "price": 10 }, { "name": "Bag", "price": 20 }]

Solution

1. Step 1: Understand the query condition

   The query uses $lt: 10, so it finds documents where price is less than 10.

2. Step 2: Check each document's price

   Only the "Pen" has price 5, which is less than 10. "Notebook" has price 10, which is not less than 10.

3. Final Answer:

   [{ "name": "Pen", "price": 5 }] -> Option C

4. Quick Check:

   Only prices less than 10 included [OK]

Hint: Remember $lt excludes equal values, only strictly less [OK]

Common Mistakes:

• Including documents with price equal to 10
• Confusing $lt with $lte
• Returning empty result by mistake

4. You wrote this query to find documents where score is less than or equal to 50:
db.scores.find({ score: { $lt: 50 } })
But you want to include documents where score is exactly 50. What is the error and how to fix it?

medium

A. Use $gt instead of $lt

B. No error, query is correct

C. Add $eq: 50 inside the query

D. Use $lte instead of $lt to include 50

Solution

1. Step 1: Identify the operator used

   The query uses $lt, which excludes values equal to 50.

2. Step 2: Choose the correct operator to include 50

   To include values equal to 50, use $lte (less than or equal).

3. Final Answer:

   Use $lte instead of $lt to include 50 -> Option D

4. Quick Check:

   $lte includes equal values [OK]

Hint: Use $lte to include the boundary value [OK]

Common Mistakes:

• Using $lt and expecting equal values included
• Using $gt which is wrong direction
• Trying to combine $lt and $eq unnecessarily

5. You have a collection orders with documents containing total values. You want to find all orders with total less than or equal to 100 but greater than 50. Which query correctly uses $lt and $lte to achieve this?

hard

A. { total: { $gte: 50, $lt: 100 } }

B. { total: { $gt: 50, $lte: 100 } }

C. { total: { $lt: 100, $lte: 50 } }

D. { total: { $lt: 100, $gt: 50 } }

Solution

1. Step 1: Understand the range needed

   We want totals greater than 50 and less than or equal to 100.

2. Step 2: Match operators to conditions

   $gt: 50 means strictly greater than 50, $lte: 100 means less than or equal to 100.

3. Step 3: Check query syntax

   { total: { $gt: 50, $lte: 100 } } correctly uses both operators in one object: { total: { $gt: 50, $lte: 100 } }.

4. Final Answer:

   { total: { $gt: 50, $lte: 100 } } -> Option B

5. Quick Check:

   Use $gt for greater than, $lte for less than or equal [OK]

Hint: Combine $gt and $lte for range queries [OK]

Common Mistakes:

• Mixing $lt and $lte incorrectly
• Using $gte instead of $gt when strict greater needed
• Reversing operator directions