$or operator behavior in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Understanding $or with multiple conditions

Given a collection products with documents:

{ "name": "Pen", "price": 5, "stock": 100 }

{ "name": "Notebook", "price": 15, "stock": 0 }

{ "name": "Eraser", "price": 3, "stock": 50 }

What documents will be returned by this query?

{ $or: [ { price: { $lt: 5 } }, { stock: { $eq: 0 } } ] }

A Documents with name "Pen" and "Eraser"

B Documents with name "Pen" and "Notebook"

C Documents with name "Notebook" and "Eraser"

D All three documents

💡 Hint

Remember $or returns documents matching at least one condition.

The query matches documents where price is less than 5 OR stock equals 0. "Eraser" has price 3 (<5), "Notebook" has stock 0, "Pen" matches neither.

🧠 Conceptual

intermediate

Behavior of $or with empty array

What happens if you run a MongoDB query with { $or: [] }?

A Returns all documents in the collection

B Returns no documents

C Throws a syntax error

D Returns documents matching all conditions

💡 Hint

Think about what it means to match any condition when there are none.

An empty $or means no conditions to satisfy, so no documents match. It returns zero documents.

📝 Syntax

advanced

Identify the invalid $or query syntax

Which of the following MongoDB queries using $or is syntactically invalid?

A { $or: { age: { $lt: 20 }, city: "NY" } }

B { $or: [ { age: { $gt: 30 } }, { name: "Alice" } ] }

C { $or: [ { score: { $gte: 90 } }, { grade: "A" } ] }

D { $or: [ { active: true }, { verified: false } ] }

💡 Hint

$or expects an array of conditions, not an object.

Option A uses an object instead of an array for $or, which is invalid syntax.

❓ optimization

advanced

Optimizing $or queries with indexes

You have a collection with indexes on status and priority. Which $or query will best use these indexes?

A { $or: [ { status: "open" }, { priority: { $gt: 5 } } ] }

B { $or: [ { status: "open", priority: { $gt: 5 } } ] }

C { $or: [ { status: "open" }, { category: "urgent" } ] }

D { $or: [ { category: "urgent" }, { priority: { $gt: 5 } } ] }

💡 Hint

Indexes help when the fields in the query match the indexed fields.

Option A uses $or with conditions on indexed fields status and priority, allowing index use for both parts.

🔧 Debug

expert

Why does this $or query return unexpected results?

Consider this query on a collection of users:

{ $or: [ { age: { $gt: 25 } }, { age: { $lt: 20 } } ] }

It returns all users, including those with age 22. Why?

A Because the $or conditions cover all ages except 20 to 25, so 22 is excluded

B Because the query matches users with age greater than 25 OR less than 20, so age 22 does not match and should not be returned

C Because the query matches users with age greater than 25 OR less than 20, so age 22 does not match either condition

D Because the query matches users with age greater than 25 OR less than 20, but the field age is stored as a string causing unexpected matches

💡 Hint

Check the data type of the age field in the documents.

If age is stored as a string, comparisons like $gt and $lt behave lexicographically, causing unexpected matches like '22' being treated differently.

Practice

1.

What does the $or operator do in MongoDB queries?

easy

A. It returns documents that match at least one of the given conditions.

B. It returns documents that match all given conditions simultaneously.

C. It returns documents that do not match any of the given conditions.

D. It sorts documents based on multiple fields.

Solution

1. Step 1: Understand the purpose of $or

   The $or operator is used to find documents that satisfy at least one condition from multiple conditions.

2. Step 2: Compare with other operators

   Unlike $and, which requires all conditions to be true, $or requires only one condition to be true.

3. Final Answer:

   It returns documents that match at least one of the given conditions. -> Option A

4. Quick Check:

   $or = match any condition [OK]

Hint: Think 'or' means any one condition true [OK]

Common Mistakes:

• Confusing $or with $and operator
• Thinking $or filters documents matching all conditions
• Assuming $or sorts documents

2.

Which of the following is the correct syntax to use $or in a MongoDB query?

{ $or: [ { age: { $lt: 20 } }, { city: "NY" } ] }

easy

A. { $or: { age: { $lt: 20 }, city: "NY" } }

B. { $or: [ age: { $lt: 20 }, city: "NY" ] }

C. { $or: [ { age: { $lt: 20 } }, { city: "NY" } ] }

D. { $or: ( age: { $lt: 20 }, city: "NY" ) }

Solution

1. Step 1: Check the structure of $or

   The $or operator requires an array of condition objects inside square brackets.

2. Step 2: Validate each option's syntax

   { $or: [ { age: { $lt: 20 } }, { city: "NY" } ] } correctly uses an array with two objects. Options A, B, and C use incorrect brackets or missing array syntax.

3. Final Answer:

   { $or: [ { age: { $lt: 20 } }, { city: "NY" } ] } -> Option C

4. Quick Check:

   $or needs array of conditions [OK]

Hint: Remember: $or takes an array of condition objects [OK]

Common Mistakes:

• Using curly braces instead of square brackets for conditions
• Using parentheses instead of brackets
• Not wrapping conditions inside an array

3.

Given the collection users with documents:

[{ "name": "Alice", "age": 25, "city": "NY" }, { "name": "Bob", "age": 19, "city": "LA" }, { "name": "Carol", "age": 30, "city": "SF" }]

What will the query db.users.find({ $or: [ { age: { $lt: 20 } }, { city: "SF" } ] }) return?

medium

A. [{ "name": "Bob", "age": 19, "city": "LA" }, { "name": "Carol", "age": 30, "city": "SF" }]

B. [{ "name": "Alice", "age": 25, "city": "NY" }]

C. [{ "name": "Bob", "age": 19, "city": "LA" }]

D. []

Solution

1. Step 1: Identify documents matching each condition

   Condition 1: age < 20 matches Bob (age 19). Condition 2: city = "SF" matches Carol.

2. Step 2: Combine results with $or

   The query returns documents matching either condition, so Bob and Carol are included.

3. Final Answer:

   [{ "name": "Bob", "age": 19, "city": "LA" }, { "name": "Carol", "age": 30, "city": "SF" }] -> Option A

4. Quick Check:

   $or returns any matching document [OK]

Hint: Check each condition separately, then combine results [OK]

Common Mistakes:

• Including documents that don't match any condition
• Confusing $or with $and and expecting all conditions to match
• Ignoring one of the conditions

4.

Consider this query that causes an error:

db.collection.find({ $or: { age: { $gt: 30 }, city: "NY" } })

What is the main issue causing the error?

medium

A. The field names inside $or must be strings.

B. The query is missing a closing parenthesis.

C. The operator $gt cannot be used inside $or.

D. $or requires an array of conditions, not a single object.

Solution

1. Step 1: Analyze the $or syntax

   $or expects an array of condition objects, but here it is given a single object.

2. Step 2: Identify the error cause

   Because the value is not an array, MongoDB throws a syntax error.

3. Final Answer:

   $or requires an array of conditions, not a single object. -> Option D

4. Quick Check:

   $or needs array syntax [OK]

Hint: Always wrap $or conditions in square brackets [OK]

Common Mistakes:

• Using object instead of array for $or conditions
• Misplacing operators inside $or
• Ignoring syntax errors from missing brackets

5.

You want to find documents in a products collection where the category is either "electronics" or the price is less than 100. Which query correctly uses $or to achieve this?

hard

A. db.products.find({ category: "electronics" && price: { $lt: 100 } })

B. db.products.find({ $or: [ { category: "electronics" }, { price: { $lt: 100 } } ] })

C. db.products.find({ $or: { category: "electronics", price: { $lt: 100 } } })

D. db.products.find({ $and: [ { category: "electronics" }, { price: { $lt: 100 } } ] })

Solution

1. Step 1: Understand the query goal

   We want documents where category is "electronics" OR price is less than 100.

2. Step 2: Check each option's logic and syntax

   db.products.find({ $or: [ { category: "electronics" }, { price: { $lt: 100 } } ] }) correctly uses $or with an array of two conditions. db.products.find({ $or: { category: "electronics", price: { $lt: 100 } } }) uses an object instead of array, causing syntax error. db.products.find({ category: "electronics" && price: { $lt: 100 } }) uses invalid syntax with &&. db.products.find({ $and: [ { category: "electronics" }, { price: { $lt: 100 } } ] }) uses $and, which requires both conditions to be true, not either.

3. Final Answer:

   db.products.find({ $or: [ { category: "electronics" }, { price: { $lt: 100 } } ] }) -> Option B

4. Quick Check:

   $or with array matches either condition [OK]

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

Common Mistakes:

• Using object instead of array for $or
• Confusing $or with $and
• Using invalid logical operators like && in MongoDB queries