Sorting by multiple fields in MongoDB

Introduction

Sorting by multiple fields helps you organize data clearly when one field alone is not enough. It lets you order results by more than one rule.

You want to list students first by grade, then by name alphabetically.

You need to show products sorted by category and then by price within each category.

You want to display events sorted by date and then by start time.

You want to order employees by department and then by their joining date.

Syntax

MongoDB

db.collection.find().sort({ field1: 1, field2: -1 })

Use 1 for ascending order and -1 for descending order.

The order of fields in the sort object matters; sorting happens in that sequence.

Examples

Sort students by grade ascending, then by name ascending.

MongoDB

db.students.find().sort({ grade: 1, name: 1 })

Sort products by category ascending, then by price descending.

MongoDB

db.products.find().sort({ category: 1, price: -1 })

Sort events by date ascending, then by start time ascending.

MongoDB

db.events.find().sort({ date: 1, startTime: 1 })

Sample Program

This inserts four employees and then sorts them by department ascending, and within each department by join year ascending.

MongoDB

db.employees.insertMany([
  { name: "Alice", department: "HR", joinYear: 2020 },
  { name: "Bob", department: "HR", joinYear: 2019 },
  { name: "Charlie", department: "IT", joinYear: 2021 },
  { name: "David", department: "IT", joinYear: 2019 }
])

const sortedEmployees = db.employees.find().sort({ department: 1, joinYear: 1 }).toArray()

sortedEmployees

Important Notes

Sorting by multiple fields is like sorting a phone book by last name, then first name.

If two documents have the same value in the first field, the second field decides their order.

Summary

Sorting by multiple fields lets you organize data with more detail.

Use 1 for ascending and -1 for descending order for each field.

The order of fields in the sort object controls the sorting priority.

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