$eq for equality in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find documents where age equals 30

Given a collection users with documents containing an age field, which query returns all users whose age is exactly 30?

MongoDB

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

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

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

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

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

💡 Hint

Use $eq to match the exact value and type.

Option B correctly uses $eq to match age exactly as number 30. Option B uses string "30" which won't match number 30. Option B works but does not explicitly use $eq operator. Option B has wrong syntax for $eq usage.

📝 Syntax

intermediate

Identify the syntax error in $eq usage

Which of the following MongoDB queries has a syntax error when using $eq?

A db.collection.find({ $eq: { field: 10 } })

B db.collection.find({ field: { $eq: "value" } })

C db.collection.find({ field: { $eq: 10 } })

D db.collection.find({ field: 10 })

💡 Hint

Check the position of $eq operator in the query.

Option A incorrectly places $eq as the top-level key with a nested field, which is invalid syntax. $eq must be inside the field's condition object.

❓ query_result

advanced

Find documents with nested field equal to a value

Given documents with a nested field address.city, which query returns documents where city equals "New York"?

A db.collection.find({ address: { city: { $eq: "New York" } } })

B db.collection.find({ "address.city": "New York" })

C db.collection.find({ "address.city": { $eq: "New York" } })

D db.collection.find({ $eq: { "address.city": "New York" } })

💡 Hint

Use dot notation to query nested fields with $eq.

Option C correctly uses dot notation with $eq to match nested field. Option C incorrectly nests $eq inside an object without dot notation. Option C works but does not explicitly use $eq. Option C has invalid syntax.

❓ optimization

advanced

Optimize query using $eq for exact match

Which query is the most efficient to find documents where status equals "active"?

A db.collection.find({ status: { $in: ["active"] } })

B db.collection.find({ status: { $eq: "active" } })

C db.collection.find({ $eq: { status: "active" } })

D db.collection.find({ status: "active" })

💡 Hint

Consider the simplest form for exact equality in MongoDB.

Option D is the simplest and most efficient query for exact match. Option D uses $eq but adds unnecessary complexity. Option D has invalid syntax. Option D uses $in which is less efficient for single value match.

🧠 Conceptual

expert

Understanding $eq behavior with different data types

What will be the result of this query if the collection has documents with score as both string and number types?

db.collection.find({ score: { $eq: 10 } })

A Returns documents where score is the number 10 only

B Returns documents where score is the string "10" only

C Returns documents where score is either number 10 or string "10"

D Returns no documents because of type mismatch

💡 Hint

MongoDB $eq matches both value and type exactly.

The $eq operator matches both value and type. So only documents with score as number 10 match, not string "10".