Question

If specified, the output of the $group stage can contain ___ that hold the values of some accumulator expression grouped by the $group's ___.

Answer 1

The $group stage in MongoDB's aggregation pipeline allows grouping of documents by a specified field and can contain fields that hold values of accumulator expressions such as sums and averages.

Step-by-step explanation:

Within the MongoDB aggregation pipeline, the $group stage is crucial for aggregating documents. The output of the $group stage can contain fields that hold the values of some accumulator expressions grouped by the $group's _id. The _id field here is often referred to as the 'group by' field, as it dictates how documents are grouped together. Accumulator expressions can then perform operations like summing values or calculating averages, allowing a wide range of statistical and analytical results to be produced from the grouped data.

For instance, if a collection contained documents with sales data, a $group stage could be used to calculate total sales per product by grouping on the product ID. Here is an example:

• Product ID (the group by criterion)
• Total sales (the accumulated sum)

This allows for powerful data analysis within MongoDB queries using the aggregation framework.