MongoDB Aggregation Framework Flashcards

Which aggregation stage filters documents to pass only those matching specified conditions to the next stage?

Answer: $match

$match acts like a WHERE clause, filtering documents based on specified criteria before passing them to the next pipeline stage.

What is the primary purpose of the $group stage in aggregation?

Answer: To group documents by a specified identifier and compute aggregate values

$group groups input documents by a specified _id expression and can apply accumulator expressions like $sum and $avg.

Which aggregation accumulator returns the sum of numeric values in a group?

Answer: $sum

$sum returns the sum of all numeric values, and when used as $sum: 1, it counts the number of documents in the group.

What does the $project stage do in an aggregation pipeline?

Answer: Passes documents with requested fields to the next stage

$project reshapes each document by including, excluding, or adding fields before passing documents to the next stage.

Which stage limits the number of documents passed to the next aggregation stage?

Answer: $limit

$limit passes only the specified number of documents to the next stage, useful for top-N queries.

What is the purpose of the $unwind stage in aggregation?

Answer: Deconstructs an array field to output one document per array element

$unwind deconstructs an array field, creating a separate output document for each array element.