Sorting by multiple fields in MongoDB - Step-by-Step Execution

Concept Flow - Sorting by multiple fields

Start Query

↓

Specify Sort Fields

↓

Sort by First Field

↓

Sort by Second Field

↓

Return Sorted Results

The query starts, specifies multiple fields to sort by, sorts first by the primary field, then by the secondary field, and finally returns the sorted results.

Execution Sample

MongoDB

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

This query sorts documents first by 'age' ascending, then by 'name' descending.

Execution Table

Step	Action	Sort Field	Sort Order	Intermediate Result
1	Start query	-	-	Unsorted documents
2	Apply sort on 'age'	age	ascending (1)	Documents ordered by age ascending
3	Apply sort on 'name' within same age	name	descending (-1)	Documents ordered by age ascending, name descending
4	Return sorted results	-	-	Final sorted documents

💡 Sorting complete, results returned sorted by age ascending and name descending.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
documents	unsorted	sorted by age ascending	sorted by age ascending and name descending	sorted by age ascending and name descending

Key Moments - 2 Insights

Why does the order of fields in the sort object matter?

Because MongoDB sorts first by the first field, then breaks ties using the second field. See execution_table rows 2 and 3.

What does 1 and -1 mean in the sort object?

1 means ascending order, -1 means descending order. This is shown in execution_table rows 2 and 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the sort order applied to the 'name' field at step 3?

A Ascending (1)

B No sort applied

C Descending (-1)

D Random order

💡 Hint

Check the 'Sort Order' column in execution_table row 3.

At which step are the documents first sorted by 'age'?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' and 'Sort Field' columns in execution_table.

If you switch the order of fields in sort to {name: -1, age: 1}, what changes in the sorting?

A Documents sort first by name descending, then age ascending

B Sorting order stays the same

C Documents sort first by age ascending, then name descending

D Sorting fails with error

💡 Hint

Refer to key_moments about the importance of field order in sorting.

Concept Snapshot

MongoDB sort syntax: .sort({field1: order1, field2: order2})
Order 1 = ascending, -1 = descending
Sorts first by field1, then breaks ties by field2
Order of fields matters
Returns documents sorted by multiple fields

Full Transcript

This visual execution shows how MongoDB sorts documents by multiple fields. The query starts unsorted, then sorts first by the primary field 'age' ascending, then by the secondary field 'name' descending. The order of fields in the sort object is important because MongoDB applies sorting in that sequence. The execution table tracks each step and the intermediate results. The variable tracker shows how the documents change from unsorted to fully sorted. Key moments clarify common confusions about sort order values and field order. The quiz tests understanding of these steps and effects.

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