$sum accumulator in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $sum accumulator do in MongoDB aggregation?

It adds up all the numeric values from the documents in a group and returns the total sum.

beginner

How do you use $sum to count the number of documents in a group?

Use $sum: 1 inside $group stage to add 1 for each document, effectively counting them.

intermediate

Can $sum be used with non-numeric fields?

No, $sum only works with numeric values. Non-numeric values will cause errors or be ignored.

beginner

Example: What does this aggregation do?

{ $group: { _id: "$category", total: { $sum: "$price" } } }

It groups documents by the category field and calculates the total sum of the price field for each category.

intermediate

What happens if you use $sum with a field that sometimes is missing in documents?

Missing fields are treated as null and do not add to the sum. Only existing numeric values are summed.

What is the purpose of the $sum accumulator in MongoDB?

A To sort documents by a field

B To find the average of numeric values

C To add numeric values in grouped documents

D To count the number of fields in a document

$sum adds numeric values from documents in a group to get a total sum.

How can you count documents using $sum in an aggregation?

A Use <code>$sum: 1</code>

B Use <code>$sum: "$field"</code>

C Use <code>$sum: null</code>

D Use <code>$sum: true</code>

Using $sum: 1 adds 1 for each document, counting them.

What happens if $sum is used on a non-numeric field?

A It throws an error or ignores non-numeric values

B It sums the values as strings

C It converts values to numbers automatically

D It counts the number of documents

$sum requires numeric values; non-numeric values cause errors or are ignored.

In this aggregation, what does { $sum: "$price" } do?<br>

{ $group: { _id: "$category", total: { $sum: "$price" } } }

A Counts documents per category

B Finds the max price per category

C Sorts categories by price

D Sums the price values per category

It sums the price field values for each category.

If some documents lack the field used in $sum, what happens?

A Missing fields count as zero in the sum

B Missing fields are ignored and do not add to the sum

C The aggregation fails

D The sum includes null values

Missing fields are treated as null and do not add to the sum.

Explain how the $sum accumulator works in MongoDB aggregation and give an example use case.

Think about adding numbers or counting items in groups.

What should you be careful about when using $sum with fields that might be missing or non-numeric?

Consider data types and missing data.