Combining logical and comparison operators in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents matching combined conditions

Given a collection products with fields price and category, which query returns all products with price greater than 100 and category either 'electronics' or 'appliances'?

MongoDB

db.products.find({ $and: [ { price: { $gt: 100 } }, { category: { $in: ['electronics', 'appliances'] } } ] })

A db.products.find({ price: { $gte: 100 }, category: { $nin: ['electronics', 'appliances'] } })

B db.products.find({ $or: [ { price: { $gt: 100 } }, { category: { $in: ['electronics', 'appliances'] } } ] })

C db.products.find({ $and: [ { price: { $lt: 100 } }, { category: { $in: ['electronics', 'appliances'] } } ] })

D db.products.find({ price: { $gt: 100 }, category: { $in: ['electronics', 'appliances'] } })

💡 Hint

Use $and to combine conditions that must both be true.

Option D uses implicit $and by specifying both conditions in the same object, which matches documents where price is greater than 100 and category is in the given list.

Option D uses $or, which matches documents where either condition is true, so it returns more documents.

Option D uses $lt (less than) for price, which is opposite of the requirement.

Option D uses $gte and $nin, which does not match the required conditions.

📝 Syntax

intermediate

Identify the syntax error in this MongoDB query

Which option contains a syntax error in combining logical and comparison operators to find users aged between 18 and 30 inclusive?

MongoDB

db.users.find({ age: { $gte: 18, $lte: 30 } })

A db.users.find({ age: { $gte: 18, $lte: 30 } })

B db.users.find({ $and: [ { age: { $gte: 18 } }, { age: { $lte: 30 } } ] })

C db.users.find({ age: { $gte: 18 }, age: { $lte: 30 } })

D db.users.find({ age: { $gte: 18, $lt: 30 } })

💡 Hint

Check if the object keys are repeated in the same level.

Option C repeats the age key twice in the same object, which is invalid syntax in MongoDB queries.

Option C correctly combines $gte and $lte in one object.

Option C uses $and with two separate conditions, which is valid.

Option C uses $gte and $lt correctly in one object.

❓ optimization

advanced

Optimize query combining multiple logical operators

Which query is the most efficient to find documents where status is 'active' and either score is above 80 or rank is below 10?

A db.records.find({ $and: [ { status: 'active' }, { $or: [ { score: { $gt: 80 } }, { rank: { $lt: 10 } } ] } ] })

B db.records.find({ status: 'active', $or: [ { score: { $gt: 80 } }, { rank: { $lt: 10 } } ] })

C db.records.find({ $or: [ { status: 'active', score: { $gt: 80 } }, { status: 'active', rank: { $lt: 10 } } ] })

D db.records.find({ $or: [ { status: 'active' }, { score: { $gt: 80 } }, { rank: { $lt: 10 } } ] })

💡 Hint

Use implicit $and by combining conditions in one object when possible.

Option B uses implicit $and by specifying status and $or conditions together, which is efficient and clear.

Option B uses explicit $and with $or, which works but is more verbose.

Option B repeats status inside each $or condition, which is redundant and less efficient.

Option B uses $or with status alone, which returns documents with status 'active' or score > 80 or rank < 10, not matching the requirement.

🔧 Debug

advanced

Debug why this query returns no results

A query is intended to find documents where age is between 20 and 40 and verified is true, but it returns no results. Which option explains the likely cause?

MongoDB

db.users.find({ $and: [ { age: { $gt: 20, $lt: 40 } }, { verified: true } ] })

A The query should use $gte and $lte to include ages 20 and 40; using $gt and $lt excludes boundary ages.

B The $and operator is invalid syntax in MongoDB queries, causing no results.

C The query uses $gt and $lt, so it excludes age 20 and 40; if no documents have age strictly between 20 and 40, no results appear.

D The verified field should be a string 'true' not boolean true, so no matches occur.

💡 Hint

Check if the comparison operators exclude boundary values.

Option A correctly identifies that $gt and $lt exclude ages exactly 20 and 40, so if documents have those ages, they won't match.

Option A is partially correct but less precise than C.

Option A is false; $and is valid syntax.

Option A assumes verified is string, but usually boolean true is correct.

🧠 Conceptual

expert

Understanding precedence of logical operators in MongoDB queries

Consider the query: db.collection.find({ $or: [ { status: 'A' }, { $and: [ { qty: { $lt: 30 } }, { price: { $gt: 20 } } ] } ] }). Which statement best describes how MongoDB evaluates this query?

A MongoDB returns documents where status is 'A' or both qty is less than 30 and price is greater than 20.

B MongoDB returns documents where status is 'A' and either qty is less than 30 or price is greater than 20.

C MongoDB returns documents where status is 'A' and qty is less than 30 and price is greater than 20.

D MongoDB returns documents where status is 'A' or qty is less than 30 or price is greater than 20.

💡 Hint

Recall that $and has higher precedence inside $or arrays.

The query uses $or with two conditions: one is status 'A', the other is an $and condition combining qty and price.

MongoDB returns documents matching either condition: status 'A' OR both qty < 30 AND price > 20.

Option A correctly describes this logic.

Options A, C, and D misunderstand the grouping and operator precedence.

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