$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".

Practice

1. What does the $eq operator do in a MongoDB query?

easy

A. It sorts documents in ascending order.

B. It deletes documents from the collection.

C. It updates documents with new values.

D. It matches documents where a field is equal to a specified value.

Solution

1. Step 1: Understand the purpose of $eq

   The $eq operator is used to filter documents where a field exactly matches a given value.

2. Step 2: Compare with other options

   Sorting, updating, and deleting are different operations and not related to $eq.

3. Final Answer:

   It matches documents where a field is equal to a specified value. -> Option D

4. Quick Check:

   $eq means equality match [OK]

Hint: Remember: $eq means 'equals' in queries [OK]

Common Mistakes:

• Confusing $eq with sorting or updating operators
• Thinking $eq modifies data instead of filtering
• Assuming $eq works for inequality

2. Which of the following is the correct syntax to find documents where the field age equals 30 using $eq?

easy

A. { age: { $eq: 30 } }

B. { $eq: { age: 30 } }

C. { age: $eq: 30 }

D. { age == 30 }

Solution

1. Step 1: Recall MongoDB query syntax for $eq

   The correct syntax uses the field name as key and an object with $eq as key and the value to match as value: { age: { $eq: 30 } }.

2. Step 2: Identify incorrect syntax

   Options B, C, and D do not follow MongoDB query syntax rules and will cause errors.

3. Final Answer:

   { age: { $eq: 30 } } -> Option A

4. Quick Check:

   Field: { $eq: value } format [OK]

Hint: Use { field: { $eq: value } } for equality [OK]

Common Mistakes:

• Placing $eq outside the field key
• Using double colons or wrong operators
• Using JavaScript equality syntax instead of MongoDB

3. Given the collection users with documents:

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

What will the query db.users.find({ age: { $eq: 30 } }) return?

medium

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

B. [] (empty array)

C. [{ "name": "Alice", "age": 25 }]

D. Syntax error

Solution

1. Step 1: Understand the query condition

   The query looks for documents where age equals 30.

2. Step 2: Check documents matching the condition

   Bob and Carol have age 30, so both documents match and will be returned.

3. Final Answer:

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

4. Quick Check:

   age == 30 returns Bob and Carol [OK]

Hint: Look for documents where field equals value [OK]

Common Mistakes:

• Returning documents with age not equal to 30
• Expecting only one document instead of all matches
• Thinking $eq causes syntax error

4. You wrote the query db.products.find({ price: $eq: 100 }) but it gives an error. What is wrong?

medium

A. Field name price is invalid

B. Missing curly braces around $eq: 100 value

C. You cannot use $eq with numbers

D. The collection name products is incorrect

Solution

1. Step 1: Check the query syntax

   The $eq operator must be inside an object as the value for the field key, like { price: { $eq: 100 } }.

2. Step 2: Identify the missing braces

   The query is missing curly braces around $eq: 100, causing a syntax error.

3. Final Answer:

   Missing curly braces around $eq: 100 value -> Option B

4. Quick Check:

   Use { field: { $eq: value } } syntax [OK]

Hint: Always wrap $eq and value in braces { } [OK]

Common Mistakes:

• Omitting braces around $eq operator
• Assuming $eq works without object syntax
• Blaming collection or field name incorrectly

5. You want to find documents in orders where the status is exactly "shipped" and the quantity is 10. Which query correctly uses $eq for both conditions?

hard

A. db.orders.find({ status: "shipped", quantity: 10 })

B. db.orders.find({ $eq: { status: "shipped", quantity: 10 } })

C. db.orders.find({ status: { $eq: "shipped" }, quantity: { $eq: 10 } })

D. db.orders.find({ status == "shipped", quantity == 10 })

Solution

1. Step 1: Use $eq for each field separately

   Each field must have its own $eq operator inside an object: { status: { $eq: "shipped" } } and { quantity: { $eq: 10 } }.

2. Step 2: Combine conditions in one query object

   Put both conditions inside the find query object to filter documents matching both.

3. Final Answer:

   db.orders.find({ status: { $eq: "shipped" }, quantity: { $eq: 10 } }) -> Option C

4. Quick Check:

   Use separate $eq for each field [OK]

Hint: Use separate $eq for each field in the query [OK]

Common Mistakes:

• Trying to use one $eq for multiple fields
• Using JavaScript equality operators in query
• Omitting $eq and relying on implicit equality