$min and $max accumulators in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Find the minimum price in the products collection

Given a products collection with documents containing a price field, which aggregation query returns the minimum price?

MongoDB

db.products.aggregate([
  { $group: { _id: null, minPrice: { $min: "$price" } } }
])

A [{ "_id": null, "minPrice": 100 }]

B [{ "_id": null, "maxPrice": 100 }]

C [{ "_id": null, "minPrice": 5 }]

D [{ "_id": null, "minPrice": null }]

💡 Hint

Use $group stage with $min accumulator on the price field.

The $min accumulator finds the smallest value of the specified field across all documents grouped. Here, grouping by null aggregates all documents together.

❓ query_result

intermediate

Find the maximum score per player

Given a scores collection with documents containing player and score fields, which aggregation query returns the highest score for each player?

MongoDB

db.scores.aggregate([
  { $group: { _id: "$player", maxScore: { $max: "$score" } } }
])

A [{ "_id": "Alice", "maxScore": 95 }, { "_id": "Bob", "maxScore": 88 }]

B [{ "_id": "Alice", "maxScore": 88 }, { "_id": "Bob", "maxScore": 95 }]

C [{ "_id": null, "maxScore": 95 }]

D [{ "_id": "Alice", "minScore": 88 }, { "_id": "Bob", "minScore": 95 }]

💡 Hint

Group by player and use $max on score.

The $max accumulator returns the highest value per group. Grouping by player gives max score per player.

📝 Syntax

advanced

Identify the syntax error in this aggregation using $min

Which option contains a syntax error in the aggregation pipeline that uses $min?

MongoDB

db.orders.aggregate([
  { $group: { _id: "$customer", minOrder: { $min: "amount" } } }
])

A SyntaxError: Missing comma after _id field

B No error, runs correctly

C TypeError: $min requires numeric values only

D SyntaxError: Field path must be prefixed with '$' inside $min accumulator

💡 Hint

Check if the field name inside $min is correctly referenced.

Field names inside accumulators must be prefixed with $. Here, "amount" should be "$amount".

❓ optimization

advanced

Optimize aggregation to find min and max in one pass

You want to find both the minimum and maximum rating in the reviews collection. Which aggregation pipeline is the most efficient?

A db.reviews.aggregate([{ $group: { _id: null, minRating: { $min: "$rating" } } }, { $group: { _id: null, maxRating: { $max: "$rating" } } }])

B db.reviews.aggregate([{ $group: { _id: null, minRating: { $min: "$rating" }, maxRating: { $max: "$rating" } } }])

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D db.reviews.aggregate([{ $group: { _id: null, minRating: { $min: "$rating" } } }, { $project: { maxRating: { $max: "$rating" } } }])

💡 Hint

Try to compute both min and max in a single $group stage.

Using one $group stage with both $min and $max is more efficient than multiple stages.

🧠 Conceptual

expert

Behavior of $min and $max with missing fields

Consider documents in a collection where some documents lack the score field. What is the behavior of $min and $max accumulators when grouping on score?

A <p>Documents missing <code>score</code> are ignored in <code>$min</code> and <code>$max</code> calculations.</p>

B <p>Documents missing <code>score</code> cause <code>$min</code> and <code>$max</code> to return null.</p>

C <p>Documents missing <code>score</code> are treated as zero in <code>$min</code> and <code>$max</code>.</p>

D <p>Documents missing <code>score</code> cause the aggregation to fail with an error.</p>

💡 Hint

Think about how MongoDB handles missing fields in accumulators.

MongoDB ignores documents where the field is missing or null when calculating $min and $max. They do not cause errors or treat missing as zero.