Sorting by multiple fields in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Sorting documents by age ascending then name descending

Given a collection people with documents containing name and age, what is the output of this query sorting by age ascending and name descending?

MongoDB

db.people.find().sort({age: 1, name: -1})

A [{name: "Zara", age: 25}, {name: "Anna", age: 25}, {name: "Bob", age: 30}]

B [{name: "Bob", age: 30}, {name: "Anna", age: 25}, {name: "Zara", age: 25}]

C [{name: "Anna", age: 25}, {name: "Zara", age: 25}, {name: "Bob", age: 30}]

D [{name: "Anna", age: 25}, {name: "Bob", age: 30}, {name: "Zara", age: 25}]

💡 Hint

Remember ascending means smallest to largest, descending means reverse alphabetical order.

The query sorts first by age ascending (25 before 30), then by name descending (Zara before Anna for age 25).

📝 Syntax

intermediate

Identify the syntax error in sorting by multiple fields

Which option contains a syntax error in the MongoDB sort method for sorting by score descending and date ascending?

MongoDB

db.records.find().sort({score: -1, date: 1})

A db.records.find().sort({score: -1, date: 1})

B db.records.find().sort({score: -1; date: 1})

C )}1 :etad ,1- :erocs{(tros.)(dnif.sdrocer.bd

D db.records.find().sort({score: -1 date: 1})

💡 Hint

Check the punctuation between fields inside the sort object.

Option B uses a semicolon instead of a comma between fields, which is invalid syntax in JavaScript objects.

❓ optimization

advanced

Optimizing sorting on multiple fields with indexes

Which index will optimize the query db.orders.find().sort({customerId: 1, orderDate: -1}) best?

A An index on {orderDate: -1, customerId: 1}

B An index on {orderDate: 1, customerId: 1}

C An index on {customerId: 1, orderDate: -1}

D An index on {customerId: -1, orderDate: 1}

💡 Hint

Indexes should match the sort fields and their order/direction.

The index must start with customerId ascending and then orderDate descending to match the sort exactly.

🔧 Debug

advanced

Why does this multi-field sort not work as expected?

A developer runs db.products.find().sort({category: 1, price: 1}) but the results are not sorted by price within each category. What is the likely cause?

A The collection has no index on category and price, causing inefficient sorting.

B The sort object keys are unordered, so MongoDB ignores the second field.

C MongoDB sorts only by the first field in the sort object, ignoring others.

D The query needs a projection to include category and price for sorting.

💡 Hint

Think about how MongoDB uses indexes to optimize sorting.

Without a supporting index, MongoDB may perform an in-memory sort that can be limited or inefficient, causing unexpected order.

🧠 Conceptual

expert

Understanding sort order precedence in MongoDB

If you run db.logs.find().sort({level: -1, timestamp: 1}), which statement best describes the sorting behavior?

A Documents are sorted randomly because mixed sort directions are not allowed.

B Documents are sorted by timestamp ascending only, ignoring level.

C Documents are sorted by level ascending, then timestamp descending.

D Documents are sorted first by level descending, then by timestamp ascending within each level.

💡 Hint

The order of fields in the sort object defines precedence.

MongoDB sorts by the first field (level descending), then by the second field (timestamp ascending) within groups of the first.

Practice

1. What does sorting by multiple fields in MongoDB allow you to do?

easy

A. Organize data by more than one field in a specific order

B. Delete multiple fields from documents

C. Create new fields based on existing ones

D. Filter documents by multiple conditions

Solution

1. Step 1: Understand sorting purpose

   Sorting arranges documents in order based on field values.

2. Step 2: Recognize multiple fields effect

   Sorting by multiple fields means ordering by the first field, then by the second if the first is equal, and so on.

3. Final Answer:

   Organize data by more than one field in a specific order -> Option A

4. Quick Check:

   Sorting by multiple fields = Organize data by multiple fields [OK]

Hint: Sorting multiple fields orders by priority fields [OK]

Common Mistakes:

• Confusing sorting with filtering
• Thinking sorting creates or deletes fields
• Assuming sorting only works on one field

2. Which of the following is the correct syntax to sort documents by age ascending and name descending in MongoDB?

easy

A. db.collection.find().sort({name: 1, age: -1})

B. db.collection.find().sort({age: -1, name: 1})

C. db.collection.find().sort({age: 1, name: -1})

D. db.collection.find().sort({age: 'asc', name: 'desc'})

Solution

1. Step 1: Recall sort syntax

   MongoDB uses 1 for ascending and -1 for descending order in sort objects.

2. Step 2: Match fields and order

   Sorting by age ascending (1) and name descending (-1) matches db.collection.find().sort({age: 1, name: -1}).

3. Final Answer:

   db.collection.find().sort({age: 1, name: -1}) -> Option C

4. Quick Check:

   age:1, name:-1 syntax = db.collection.find().sort({age: 1, name: -1}) [OK]

Hint: Use 1 for ascending, -1 for descending in sort object [OK]

Common Mistakes:

• Using strings like 'asc' or 'desc' instead of 1 or -1
• Mixing order of fields incorrectly
• Using wrong signs for ascending/descending

3. Given the collection documents:

{"name": "Alice", "age": 30, "score": 85}
{"name": "Bob", "age": 25, "score": 90}
{"name": "Alice", "age": 30, "score": 95}

What is the order of documents after running db.collection.find().sort({name: 1, age: -1, score: 1})?

medium

A. [{name: "Alice", age: 30, score: 85}, {name: "Bob", age: 25, score: 90}, {name: "Alice", age: 30, score: 95}]

B. [{name: "Alice", age: 30, score: 95}, {name: "Alice", age: 30, score: 85}, {name: "Bob", age: 25, score: 90}]

C. [{name: "Bob", age: 25, score: 90}, {name: "Alice", age: 30, score: 85}, {name: "Alice", age: 30, score: 95}]

D. [{name: "Alice", age: 30, score: 85}, {name: "Alice", age: 30, score: 95}, {name: "Bob", age: 25, score: 90}]

Solution

1. Step 1: Sort by name ascending

   Names sorted ascending: "Alice" before "Bob".

2. Step 2: Sort by age descending within same name

   Both "Alice" have age 30, so order stays same.

3. Step 3: Sort by score ascending within same name and age

   Scores 85 then 95 for "Alice".

4. Final Answer:

   [{name: "Alice", age: 30, score: 85}, {name: "Alice", age: 30, score: 95}, {name: "Bob", age: 25, score: 90}] -> Option D

5. Quick Check:

   Sort by name↑, age↓, score↑ = [{name: "Alice", age: 30, score: 85}, {name: "Alice", age: 30, score: 95}, {name: "Bob", age: 25, score: 90}] [OK]

Hint: Sort priority follows field order in sort object [OK]

Common Mistakes:

• Ignoring order of fields in sort
• Mixing ascending and descending incorrectly
• Assuming score sorts descending by default

4. Identify the error in this MongoDB sort query:

db.collection.find().sort({age: 1, name: 2})

medium

A. Using 2 instead of -1 or 1 for sorting order

B. Missing parentheses after find()

C. Using curly braces instead of square brackets

D. Sorting fields must be strings, not numbers

Solution

1. Step 1: Check valid sort values

   MongoDB accepts only 1 (ascending) or -1 (descending) as sort values.

2. Step 2: Identify invalid value

   Value 2 is invalid and causes syntax error.

3. Final Answer:

   Using 2 instead of -1 or 1 for sorting order -> Option A

4. Quick Check:

   Sort values must be 1 or -1 [OK]

Hint: Sort values must be 1 or -1, never other numbers [OK]

Common Mistakes:

• Using numbers other than 1 or -1
• Confusing sort object syntax
• Assuming 2 means descending

5. You want to sort a collection by department ascending, then by salary descending, but only for employees with the same department. Which MongoDB query correctly achieves this?

hard

A. db.employees.find().sort({salary: -1, department: 1})

B. db.employees.find().sort({department: 1, salary: -1})

C. db.employees.find().sort({department: -1, salary: 1})

D. db.employees.find().sort({salary: 1, department: -1})

Solution

1. Step 1: Understand sorting priority

   Sorting by department ascending means all employees grouped by department alphabetically.

2. Step 2: Sort salary descending within each department

   Within each department group, employees are ordered by salary from highest to lowest.

3. Step 3: Match correct sort order

   db.employees.find().sort({department: 1, salary: -1}) matches department:1 (ascending) and salary:-1 (descending).

4. Final Answer:

   db.employees.find().sort({department: 1, salary: -1}) -> Option B

5. Quick Check:

   Sort by department↑ then salary↓ = db.employees.find().sort({department: 1, salary: -1}) [OK]

Hint: Order fields in sort by priority, use 1 or -1 for direction [OK]

Common Mistakes:

• Reversing field order in sort object
• Using wrong sort directions
• Assuming sorting salary first groups by salary only