$min and $max accumulators in MongoDB - Mini Project: Build & Apply

$min and $max Accumulators in MongoDB Aggregation

📖 Scenario: You work at a small bookstore that keeps track of sales in a MongoDB collection. Each sale document records the book title and the number of copies sold. You want to find the minimum and maximum number of copies sold among all sales to understand the range of sales performance.

🎯 Goal: Build a MongoDB aggregation pipeline that uses the $min and $max accumulators to find the smallest and largest number of copies sold in the sales collection.

📋 What You'll Learn

Create a sales collection with documents containing title and copies_sold fields.

Define a variable pipeline that holds the aggregation pipeline array.

Use the $group stage with $min and $max accumulators on the copies_sold field.

Complete the pipeline so it returns a single document with minCopies and maxCopies fields.

💡 Why This Matters

🌍 Real World

Bookstores and many businesses use MongoDB to analyze sales data and find minimum and maximum values to understand performance ranges.

💼 Career

Knowing how to use MongoDB aggregation with accumulators like $min and $max is important for data analysts and backend developers working with NoSQL databases.

1

Create the sales collection data

Create a variable called sales that is an array of documents with these exact entries: { title: "Book A", copies_sold: 5 }, { title: "Book B", copies_sold: 12 }, { title: "Book C", copies_sold: 7 }, { title: "Book D", copies_sold: 3 }, and { title: "Book E", copies_sold: 10 }.

MongoDB

const sales = [
  // Your code here
];

Hint

Use an array of objects with the exact field names and values given.

2

Define the aggregation pipeline variable

Create a variable called pipeline and set it to an empty array [].

MongoDB

const sales = [
  { title: "Book A", copies_sold: 5 },
  { title: "Book B", copies_sold: 12 },
  { title: "Book C", copies_sold: 7 },
  { title: "Book D", copies_sold: 3 },
  { title: "Book E", copies_sold: 10 }
];

// Your code here

Hint

Just create an empty array named pipeline for now.

3

Add the $group stage with $min and $max accumulators

Add a $group stage object to the pipeline array. Use _id: null and add minCopies: { $min: "$copies_sold" } and maxCopies: { $max: "$copies_sold" } fields inside the $group stage.

MongoDB

const sales = [
  { title: "Book A", copies_sold: 5 },
  { title: "Book B", copies_sold: 12 },
  { title: "Book C", copies_sold: 7 },
  { title: "Book D", copies_sold: 3 },
  { title: "Book E", copies_sold: 10 }
];

const pipeline = [
  // Your code here
];

Hint

Remember to use $group with _id: null to aggregate all documents together.

4

Complete the aggregation pipeline setup

Add a comment above the pipeline variable that says // Aggregation pipeline to find min and max copies sold.

MongoDB

// Your code here
const sales = [
  { title: "Book A", copies_sold: 5 },
  { title: "Book B", copies_sold: 12 },
  { title: "Book C", copies_sold: 7 },
  { title: "Book D", copies_sold: 3 },
  { title: "Book E", copies_sold: 10 }
];

const pipeline = [
  {
    $group: {
      _id: null,
      minCopies: { $min: "$copies_sold" },
      maxCopies: { $max: "$copies_sold" }
    }
  }
];

Hint

Adding a comment helps explain what the pipeline does.

Practice

1. What do the $min and $max accumulators do in a MongoDB aggregation pipeline?

easy

A. $min and $max sort documents in ascending and descending order respectively.

B. $min and $max both find the average value in a group.

C. $min finds the first document and $max finds the last document in a group.

D. $min finds the smallest value and $max finds the largest value in a group.

Solution

1. Step 1: Understand the purpose of $min and $max

   $min returns the smallest value from the grouped documents, while $max returns the largest value.

2. Step 2: Compare with other options

   The other options describe different operations like average, first/last document, or sorting, which are not what $min and $max do.

3. Final Answer:

   $min finds the smallest value and $max finds the largest value in a group. -> Option D

4. Quick Check:

   $min = smallest, $max = largest [OK]

Hint: Remember: min = smallest, max = largest value [OK]

Common Mistakes:

• Confusing $min and $max with average functions
• Thinking they return documents instead of values
• Mixing accumulators with sorting operators

2. Which of the following is the correct syntax to use $max in a $group stage to find the highest score from a field named score?

easy

A. { $group: { _id: null, maxScore: { $max: "$score" } } }

B. { $group: { _id: null, maxScore: { max: "$score" } } }

C. { $group: { _id: null, maxScore: { $max: score } } }

D. { $group: { _id: null, maxScore: { $max: "score" } } }

Solution

1. Step 1: Check the correct use of $max accumulator

   The correct syntax requires $max as the operator and the field name as a string with a $ prefix inside quotes: "$score".

2. Step 2: Identify syntax errors in other options

   { $group: { _id: null, maxScore: { max: "$score" } } } misses the $ before max, { $group: { _id: null, maxScore: { $max: score } } } lacks quotes around field name, and { $group: { _id: null, maxScore: { $max: "score" } } } uses quotes without $ prefix, all invalid.

3. Final Answer:

   { $group: { _id: null, maxScore: { $max: "$score" } } } -> Option A

4. Quick Check:

   Use $max: "$field" syntax [OK]

Hint: Always prefix field names with $ and use quotes inside accumulators [OK]

Common Mistakes:

• Omitting $ before accumulator name
• Not quoting the field name with $
• Using field name without $ or quotes

3. Given the collection documents:

{ "name": "Alice", "score": 85 }, { "name": "Bob", "score": 92 }, { "name": "Charlie", "score": 78 }

What will be the output of this aggregation?

[{ $group: { _id: null, minScore: { $min: "$score" }, maxScore: { $max: "$score" } } }]

medium

A. { "_id": null, "minScore": 85, "maxScore": 85 }

B. { "_id": null, "minScore": 78, "maxScore": 92 }

C. { "_id": null, "minScore": 78, "maxScore": 78 }

D. { "_id": null, "minScore": 92, "maxScore": 85 }

Solution

1. Step 1: Identify minimum and maximum scores from documents

   The scores are 85, 92, and 78. The smallest is 78 and the largest is 92.

2. Step 2: Understand the aggregation output

   The $group stage groups all documents (_id: null) and calculates minScore as 78 and maxScore as 92.

3. Final Answer:

   { "_id": null, "minScore": 78, "maxScore": 92 } -> Option B

4. Quick Check:

   Min = 78, Max = 92 [OK]

Hint: Scan values, pick smallest for $min and largest for $max [OK]

Common Mistakes:

• Mixing up min and max values
• Assuming $min or $max returns documents instead of values
• Forgetting to use $ before field names

4. Consider this aggregation pipeline:

[{ $group: { _id: "$category", minPrice: { $min: price }, maxPrice: { $max: "$price" } } }]

What is the error in this pipeline?

medium

A. Field name 'price' in $min is missing quotes and $ prefix.

B. Using $max and $min together is not allowed.

C. The _id field cannot be a field name.

D. The pipeline is correct and has no errors.

Solution

1. Step 1: Check syntax of $min and $max usage

   The $min accumulator uses price without quotes and $ prefix, which is incorrect. It should be "$price".

2. Step 2: Verify other parts of the pipeline

   $max is correctly used with "$price". Grouping by "$category" is valid. Using both accumulators together is allowed.

3. Final Answer:

   Field name 'price' in $min is missing quotes and $ prefix. -> Option A

4. Quick Check:

   Field names need "$" prefix inside accumulators [OK]

Hint: Always quote and prefix field names with $ inside accumulators [OK]

Common Mistakes:

• Omitting $ before field names inside accumulators
• Thinking $min and $max cannot be used together
• Misunderstanding _id field usage in $group

5. You have a collection of sales documents with fields region and amount. You want to find the smallest and largest sale amounts per region, but only for regions where the smallest sale is above 100. Which aggregation pipeline correctly achieves this?

hard

A. [{ $group: { _id: "$region", minSale: { $min: "$amount" }, maxSale: { $max: "$amount" }, filter: { $gt: ["$minSale", 100] } } }]

B. [{ $match: { amount: { $gt: 100 } } }, { $group: { _id: "$region", minSale: { $min: "$amount" }, maxSale: { $max: "$amount" } } }]

C. [{ $group: { _id: "$region", minSale: { $min: "$amount" }, maxSale: { $max: "$amount" } } }, { $match: { minSale: { $gt: 100 } } }]

D. [{ $group: { _id: "$region", minSale: { $min: "$amount" }, maxSale: { $max: "$amount" } } }, { $match: { maxSale: { $gt: 100 } } }]

Solution

1. Step 1: Group by region and calculate min and max sales

   The $group stage groups documents by region and calculates minSale and maxSale using $min and $max on amount.

2. Step 2: Filter groups where minSale is greater than 100

   The $match stage after grouping filters groups where minSale is greater than 100, exactly as required.

3. Final Answer:

   [{ $group: { _id: "$region", minSale: { $min: "$amount" }, maxSale: { $max: "$amount" } } }, { $match: { minSale: { $gt: 100 } } }] -> Option C

4. Quick Check:

   Group first, then filter on aggregated minSale [OK]

Hint: Filter after grouping to use aggregated fields in conditions [OK]

Common Mistakes:

• Filtering before grouping, which filters documents not groups
• Trying to filter inside $group stage incorrectly
• Filtering on maxSale instead of minSale