Combining logical and comparison operators in MongoDB - Mini Project: Build & Apply

Combining Logical and Comparison Operators in MongoDB Queries

📖 Scenario: You are managing a small online bookstore database. You want to find books that meet certain conditions to help with marketing and stock decisions.

🎯 Goal: Build a MongoDB query that uses both logical and comparison operators to find books that are either priced below $20 and have more than 50 copies in stock, or are authored by 'Jane Austen'.

📋 What You'll Learn

Create a collection named books with specific book documents.

Add a variable priceLimit set to 20.

Write a MongoDB query using $or, $and, $lt, $gt, and $eq operators.

Complete the query to find books matching the conditions described.

💡 Why This Matters

🌍 Real World

Filtering products or items in a database based on multiple conditions is common in e-commerce, inventory management, and reporting.

💼 Career

Knowing how to combine logical and comparison operators in queries is essential for database developers, data analysts, and backend engineers.

1

Create the books collection with sample documents

Create a MongoDB collection called books and insert these exact documents: { title: 'Pride and Prejudice', author: 'Jane Austen', price: 15, stock: 30 }, { title: '1984', author: 'George Orwell', price: 18, stock: 60 }, { title: 'The Great Gatsby', author: 'F. Scott Fitzgerald', price: 22, stock: 80 }, { title: 'Emma', author: 'Jane Austen', price: 25, stock: 40 }.

MongoDB

// Insert the specified documents into the books collection
// Your code here

Hint

Use db.books.insertMany([...]) with the exact documents inside the array.

2

Set the price limit variable

Create a variable called priceLimit and set it to the number 20.

MongoDB

db.books.insertMany([
  { title: 'Pride and Prejudice', author: 'Jane Austen', price: 15, stock: 30 },
  { title: '1984', author: 'George Orwell', price: 18, stock: 60 },
  { title: 'The Great Gatsby', author: 'F. Scott Fitzgerald', price: 22, stock: 80 },
  { title: 'Emma', author: 'Jane Austen', price: 25, stock: 40 }
])

// Create priceLimit variable and set to 20
// Your code here

Hint

Use const priceLimit = 20; to create the variable.

3

Write the MongoDB query with logical and comparison operators

Write a MongoDB query called query that uses $or with two conditions: one is an $and combining price less than priceLimit and stock greater than 50, the other is author equal to 'Jane Austen'.

MongoDB

db.books.insertMany([
  { title: 'Pride and Prejudice', author: 'Jane Austen', price: 15, stock: 30 },
  { title: '1984', author: 'George Orwell', price: 18, stock: 60 },
  { title: 'The Great Gatsby', author: 'F. Scott Fitzgerald', price: 22, stock: 80 },
  { title: 'Emma', author: 'Jane Austen', price: 25, stock: 40 }
])

const priceLimit = 20;

// Write the query using $or, $and, $lt, $gt, and $eq
// Your code here

Hint

Use $or with an array containing an $and condition and an $eq condition for author.

4

Complete the query by running it on the books collection

Add a line to run db.books.find(query) to get the matching books.

MongoDB

db.books.insertMany([
  { title: 'Pride and Prejudice', author: 'Jane Austen', price: 15, stock: 30 },
  { title: '1984', author: 'George Orwell', price: 18, stock: 60 },
  { title: 'The Great Gatsby', author: 'F. Scott Fitzgerald', price: 22, stock: 80 },
  { title: 'Emma', author: 'Jane Austen', price: 25, stock: 40 }
])

const priceLimit = 20;

const query = {
  $or: [
    { $and: [ { price: { $lt: priceLimit } }, { stock: { $gt: 50 } } ] },
    { author: { $eq: 'Jane Austen' } }
  ]
};

// Run the query on the books collection
// Your code here

Hint

Use db.books.find(query); to run the query.

Practice

1. Which MongoDB operator is used to combine multiple conditions where all must be true?

easy

A. $gt

B. $and

C. $or

D. $lt

Solution

1. Step 1: Understand logical operators

   $and requires all conditions inside it to be true for a document to match.

2. Step 2: Compare with other operators

   $or requires any condition to be true, $gt and $lt are comparison operators, not logical combiners.

3. Final Answer:

   $and -> Option B

4. Quick Check:

   All conditions true = $and [OK]

Hint: Use $and to require all conditions true [OK]

Common Mistakes:

• Confusing $and with $or
• Using comparison operators as logical
• Mixing $gt with $and

2. Which of the following is the correct syntax using $and to find documents where age is greater than 25 and status is 'active'?

easy

A. { $and: [ { age: { $gt: 25 } }, { status: 'active' } ] }

B. { $or: [ { age: { $gt: 25 } }, { status: 'active' } ] }

C. { age: { $gt: 25 }, status: 'active' }

D. { $and: { age: { $gt: 25 }, status: 'active' } }

Solution

1. Step 1: Check $and syntax

   $and requires an array of conditions, so the value must be an array of objects.

2. Step 2: Validate other options

   { $or: [ { age: { $gt: 25 } }, { status: 'active' } ] } uses $or, { age: { $gt: 25 }, status: 'active' } uses implicit AND but not with $and operator, { $and: { age: { $gt: 25 }, status: 'active' } } uses $and with an object instead of array which is invalid.

3. Final Answer:

   { $and: [ { age: { $gt: 25 } }, { status: 'active' } ] } -> Option A

4. Quick Check:

   $and needs array of conditions [OK]

Hint: Use array inside $and for multiple conditions [OK]

Common Mistakes:

• Using object instead of array in $and
• Confusing $and with $or
• Missing $gt operator syntax

3. Given the collection documents: [{ age: 30, status: 'active' }, { age: 20, status: 'inactive' }, { age: 40, status: 'active' }], what documents match this query?
{ $or: [ { age: { $lt: 25 } }, { status: 'active' } ] }

medium

A. [{ age: 30, status: 'active' }, { age: 40, status: 'active' }]

B. []

C. [{ age: 20, status: 'inactive' }]

D. [{ age: 20, status: 'inactive' }, { age: 30, status: 'active' }, { age: 40, status: 'active' }]

Solution

1. Step 1: Understand $or condition

   Documents match if age is less than 25 OR status is 'active'.

2. Step 2: Check each document

   First doc: age 30 (not <25), status 'active' (matches). Second doc: age 20 (<25), status 'inactive' (matches age condition). Third doc: age 40 (not <25), status 'active' (matches).

3. Final Answer:

   All three documents match -> Option D

4. Quick Check:

   $or matches any condition true [OK]

Hint: Any condition true in $or returns document [OK]

Common Mistakes:

• Ignoring one condition in $or
• Confusing $or with $and
• Wrongly excluding documents

4. Identify the error in this MongoDB query to find documents where score is greater than 50 and less than 100:
{ $and: { score: { $gt: 50 }, score: { $lt: 100 } } }

medium

A. Using object instead of array inside $and

B. Using $gt and $lt together is invalid

C. Missing $or operator

D. score field should be inside $or

Solution

1. Step 1: Check $and operator usage

   $and requires an array of condition objects, not a single object with repeated keys.

2. Step 2: Understand object key overwrite

   In the object, 'score' key appears twice, so only the last one is used, causing incorrect query.

3. Final Answer:

   Using object instead of array inside $and -> Option A

4. Quick Check:

   $and needs array, not object with duplicate keys [OK]

Hint: Use array for $and conditions to avoid key overwrite [OK]

Common Mistakes:

• Using object with duplicate keys in $and
• Confusing $and with $or
• Thinking $gt and $lt can't combine

5. You want to find documents where (age is greater than 30 and status is 'active') or (score is less than 50). Which query correctly combines these conditions?

hard

A. { $and: { $or: [ { age: { $gt: 30 } }, { status: 'active' } ], score: { $lt: 50 } } }

B. { $and: [ { age: { $gt: 30 } }, { status: 'active' }, { score: { $lt: 50 } } ] }

C. { $or: [ { $and: [ { age: { $gt: 30 } }, { status: 'active' } ] }, { score: { $lt: 50 } } ] }

D. { $or: { age: { $gt: 30 }, status: 'active', score: { $lt: 50 } } }

Solution

1. Step 1: Break down the condition

   We want documents matching either (age > 30 AND status = 'active') OR (score < 50).

2. Step 2: Match query structure

   { $or: [ { $and: [ { age: { $gt: 30 } }, { status: 'active' } ] }, { score: { $lt: 50 } } ] } uses $or with two parts: one $and array for age and status, and one condition for score less than 50, matching the requirement exactly.

3. Final Answer:

   { $or: [ { $and: [ { age: { $gt: 30 } }, { status: 'active' } ] }, { score: { $lt: 50 } } ] } -> Option C

4. Quick Check:

   Nested $and inside $or matches complex logic [OK]

Hint: Use nested $and inside $or for combined conditions [OK]

Common Mistakes:

• Using $and instead of $or for main condition
• Wrong object instead of array syntax
• Mixing $or and $and incorrectly