Query filter syntax in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents with age greater than 30

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

MongoDB

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

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

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

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

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

💡 Hint

Use $gt to find values greater than a number.

The $gt operator filters documents where the field value is greater than the specified number. $gte includes the number itself, $lt is less than, and matching age: 30 finds exact matches.

❓ query_result

intermediate

Find documents with name starting with 'A'

Which query returns all documents where the name field starts with the letter 'A'?

MongoDB

db.users.find({ name: /^A/ })

A db.users.find({ name: /^A/ })

B db.users.find({ name: { $regex: 'A$' } })

C db.users.find({ name: { $regex: '^A' } })

D db.users.find({ name: { $regex: 'A' } })

💡 Hint

Use a regular expression to match the start of the string.

The regex /^A/ matches strings starting with 'A'. Option A is similar but uses $regex syntax. However, option A is the direct and correct syntax. Option A matches strings ending with 'A'. Option A matches strings containing 'A' anywhere.

📝 Syntax

advanced

Identify the syntax error in this query

Which option contains a syntax error in the MongoDB query filter?

MongoDB

db.products.find({ price: { $lt: 100, $gt: 50 } })

A db.products.find({ price: { $gte: 50, $lte: 100 } })

B db.products.find({ price: { $gt: 50, $lt: 100 } })

C db.products.find({ price: { $lt: 100, $gt: 50 } })

D db.products.find({ price: { $lt: 100 } $gt: 50 })

💡 Hint

Check for missing commas or misplaced braces.

Option D is missing a comma between the two operators inside the object, causing a syntax error. Options B, C, and D are syntactically correct.

❓ optimization

advanced

Optimize query to find users with status 'active' or 'pending'

Which query is the most efficient to find users whose status is either 'active' or 'pending'?

MongoDB

db.users.find({ status: { $in: ['active', 'pending'] } })

A db.users.find({ $or: [ { status: 'active' }, { status: 'pending' } ] })

B db.users.find({ status: 'active' || 'pending' })

C db.users.find({ status: { $in: ['active', 'pending'] } })

D db.users.find({ status: { $or: ['active', 'pending'] } })

💡 Hint

Use $in operator for multiple possible values in one field.

Option C uses $in which is optimized for matching any value in the list. Option C works but is less concise. Option C is invalid syntax. Option C uses $or incorrectly inside a field.

🧠 Conceptual

expert

Understanding query filter behavior with missing fields

What will the query db.orders.find({ shippedDate: { $exists: false } }) return?

A Documents where the shippedDate field exists and is null

B Documents where the shippedDate field does not exist

C Documents where the shippedDate field is missing or null

D Documents where the shippedDate field exists and has any value

💡 Hint

The $exists operator checks if a field is present in the document.

The query with $exists: false returns documents where the field is not present at all. It does not include documents where the field exists but is null.

Practice

1. What does a MongoDB query filter do?
{ age: 25 }

easy

A. Deletes documents where age is 25

B. Updates documents to set age to 25

C. Selects documents where the age field is exactly 25

D. Creates a new document with age 25

Solution

1. Step 1: Understand the role of a query filter

   A query filter in MongoDB is used to find documents that match certain conditions, not to modify or delete them.

2. Step 2: Analyze the filter { age: 25 }

   This filter matches documents where the field age has the value 25 exactly.

3. Final Answer:

   Selects documents where the age field is exactly 25 -> Option C

4. Quick Check:

   Query filter = select matching documents [OK]

Hint: Filters pick matching data, not modify or delete [OK]

Common Mistakes:

• Confusing filter with update or delete commands
• Thinking filter creates or changes documents
• Assuming filter matches partial or range values without operators

2. Which of these is the correct syntax to find documents where score is greater than 80?

easy

A. { score > 80 }

B. { score: gt 80 }

C. { $score: { gt: 80 } }

D. { score: { $gt: 80 } }

Solution

1. Step 1: Recall MongoDB operator syntax

   MongoDB uses JSON objects with operators starting with a dollar sign, like $gt for 'greater than'.

2. Step 2: Check each option

   { score: { $gt: 80 } } uses { score: { $gt: 80 } } which is correct syntax. Others miss the dollar sign or use invalid syntax.

3. Final Answer:

   { score: { $gt: 80 } } -> Option D

4. Quick Check:

   Use $gt for greater than in filters [OK]

Hint: Operators start with $ inside field object [OK]

Common Mistakes:

• Omitting $ before operator
• Using comparison operators like > directly
• Wrong operator names without $

3. Given the collection documents:

{ name: "Alice", age: 30 }
{ name: "Bob", age: 25 }
{ name: "Carol", age: 30 }

What will this query return?
db.collection.find({ age: 30 })

medium

A. [{ name: "Alice", age: 30 }, { name: "Carol", age: 30 }]

B. [{ name: "Bob", age: 25 }]

C. [] (empty array)

D. Syntax error

Solution

1. Step 1: Understand the filter condition

   The filter { age: 30 } matches documents where the age field equals 30.

2. Step 2: Identify matching documents

   Documents for Alice and Carol have age 30, so both match. Bob has age 25, so does not match.

3. Final Answer:

   [{ name: "Alice", age: 30 }, { name: "Carol", age: 30 }] -> Option A

4. Quick Check:

   Filter matches exact age 30 documents [OK]

Hint: Filter matches exact field values without operators [OK]

Common Mistakes:

• Expecting only one document returned
• Confusing filter with update or delete
• Thinking filter matches partial or range without operators

4. What is wrong with this MongoDB query filter?
db.collection.find({ age: $gt: 20 })

medium

A. Using $gt instead of $gte

B. Missing curly braces around $gt operator

C. Field name should be $age, not age

D. No error, query is correct

Solution

1. Step 1: Check operator syntax in filter

   MongoDB requires operators like $gt to be inside an object as a value for the field key.

2. Step 2: Identify the syntax error

   The query uses { age: $gt: 20 } which is invalid. It should be { age: { $gt: 20 } } with curly braces around $gt: 20.

3. Final Answer:

   Missing curly braces around $gt operator -> Option B

4. Quick Check:

   Operators must be inside braces as field values [OK]

Hint: Put operators inside braces after field name [OK]

Common Mistakes:

• Omitting braces around operator
• Using wrong operator names
• Misplacing $ sign

5. You want to find documents where status is either "active" or "pending". Which filter correctly does this?

hard

A. { status: { $in: ["active", "pending"] } }

B. { status: { $or: ["active", "pending"] } }

C. { $or: { status: "active", status: "pending" } }

D. { status: "active" || "pending" }

Solution

1. Step 1: Understand how to match multiple values

   MongoDB uses $in operator to match a field against any value in a list.

2. Step 2: Analyze each option

   { status: { $in: ["active", "pending"] } } uses { status: { $in: ["active", "pending"] } } which is correct. { status: { $or: ["active", "pending"] } } uses invalid operator $or inside field. { $or: { status: "active", status: "pending" } } has wrong syntax for $or. { status: "active" || "pending" } uses invalid JavaScript syntax.

3. Final Answer:

   { status: { $in: ["active", "pending"] } } -> Option A

4. Quick Check:

   Use $in for matching any value in list [OK]

Hint: Use $in with array for multiple value matches [OK]

Common Mistakes:

• Using $or inside field instead of top-level
• Wrong syntax for $or operator
• Using JavaScript operators inside filter