$group stage for aggregation in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of $group with sum accumulator

Given the collection sales with documents:

{ "item": "apple", "qty": 5 }
{ "item": "banana", "qty": 10 }
{ "item": "apple", "qty": 15 }

What is the output of this aggregation?

db.sales.aggregate([{ $group: { _id: "$item", totalQty: { $sum: "$qty" } } }])

MongoDB

db.sales.aggregate([{ $group: { _id: "$item", totalQty: { $sum: "$qty" } } }])

A [{ "_id": "apple", "totalQty": 20 }, { "_id": "banana", "totalQty": 10 }]

B [{ "_id": "apple", "totalQty": 5 }, { "_id": "banana", "totalQty": 10 }, { "_id": "apple", "totalQty": 15 }]

C [{ "_id": null, "totalQty": 30 }]

D [{ "_id": "apple", "totalQty": 15 }, { "_id": "banana", "totalQty": 10 }]

💡 Hint

Think about grouping by the item name and summing quantities for each group.

The $group stage groups documents by the _id field, here the item name. Then it sums the qty values for each group. Apples have qty 5 + 15 = 20, bananas have qty 10.

📝 Syntax

intermediate

Identify the syntax error in $group stage

Which option contains a syntax error in the $group stage of this aggregation pipeline?

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

A { $group: { _id: "$customer", total: { $sum: "$amount" } } }

B { $group: { _id: "$customer", total: { $sum: amount } } }

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D $group: { _id: "$customer", total: { $sum: "$amount" } } }

💡 Hint

Check if the field names are properly quoted as strings.

In option B, $sum is given amount without quotes, which is invalid. Field names must be strings with $ prefix inside quotes.

❓ query_result

advanced

Output of $group with multiple accumulators

Given the collection products with documents:

{ "category": "fruit", "price": 10 }
{ "category": "fruit", "price": 15 }
{ "category": "vegetable", "price": 20 }

What is the output of this aggregation?

db.products.aggregate([{ $group: { _id: "$category", count: { $sum: 1 }, avgPrice: { $avg: "$price" } } }])

MongoDB

db.products.aggregate([{ $group: { _id: "$category", count: { $sum: 1 }, avgPrice: { $avg: "$price" } } }])

A [{ "_id": "fruit", "count": 2, "avgPrice": 12.5 }, { "_id": "vegetable", "count": 1, "avgPrice": 20 }]

B [{ "_id": "fruit", "count": 1, "avgPrice": 12.5 }, { "_id": "vegetable", "count": 1, "avgPrice": 20 }]

C [{ "_id": "fruit", "count": 2, "avgPrice": 25 }, { "_id": "vegetable", "count": 1, "avgPrice": 20 }]

D [{ "_id": null, "count": 3, "avgPrice": 15 }]

💡 Hint

Count counts documents per group, avgPrice averages the price field per group.

The $group groups by category. For fruit, two documents with prices 10 and 15 average to 12.5 and count is 2. Vegetable has one document with price 20.

🔧 Debug

advanced

Why does this $group aggregation return empty results?

Consider this aggregation on collection logs:

db.logs.aggregate([{ $group: { _id: "$level", total: { $sum: "$count" } } }])

But the output is an empty array. The documents look like:

{ "level": "error", "count": 5 }
{ "level": "info", "count": 3 }

What is the most likely reason for empty output?

A The field "count" does not exist in documents, so $sum returns null and no groups form.

B The aggregation pipeline is missing a $match stage to filter documents.

C The field "count" is a string, so $sum fails silently and returns empty.

D The aggregation is correct; empty output means no documents in collection.

💡 Hint

Consider if there are actually documents in the collection matching the pipeline.

Empty output from an aggregation pipeline with no filtering stages ($match, etc.) indicates that the collection has no documents. Even if fields are missing, $group would output groups with accumulators initialized to 0 or null.

🧠 Conceptual

expert

Understanding $group _id field behavior

In MongoDB aggregation, what happens if you set _id to null in the $group stage?

Choose the correct explanation:

A Documents are grouped by a field named "null", which usually does not exist, so output is empty.

B Each document is grouped by its own _id field, so output has same number of documents as input.

C All documents are grouped into a single group, so the output has one document aggregating all input documents.

D The aggregation fails with an error because _id cannot be null in $group.

💡 Hint

Think about grouping all documents together regardless of any field.

Setting _id to null groups all documents into one group, producing a single output document with aggregated values over the entire collection.