MongoDB MongoDB Aggregation Framework 2

Question 1

Which aggregation stage performs a left outer join to another collection?

Accepted Answer

$lookup

Answer

$lookup performs a left outer join with another collection in the same database, adding matched documents as an array field.

Question 2

What does the $avg accumulator compute in a $group stage?

Accepted Answer

The arithmetic mean of numeric values

Answer

$avg calculates the average of all numeric values for documents in each group.

Question 3

Which stage sorts the documents in the aggregation pipeline?

Accepted Answer

$sort

Answer

$sort reorders pipeline documents according to specified fields, using 1 for ascending and -1 for descending.

Question 4

What is the $addFields stage used for in aggregation?

Accepted Answer

Adds new fields to documents without removing existing ones

Answer

$addFields outputs documents containing all existing fields plus any newly added or modified fields.

Question 5

Which aggregation stage writes the results of the pipeline to a specified collection?

Accepted Answer

$out

Answer

$out writes the resulting documents to a specified collection, replacing it if it already exists.

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