What does the following aggregate query perform?

Question

Options

• AFetches the posts with likes between 100 and 200 and sets their _id as null
• BGroups the posts by number of likes (101, 102, 103.) by adding 1 every time
• CCalculates the number of posts with likes between 100 and 200
• DFetches the posts with likes between 100 and 200, sets the _id of the first document as null and

• A
  3% (1)
• B
  7% (2)
• C
  76% (22)
• D
  14% (4)

Explanation

Option C is correct because the pipeline uses $match to filter posts where likes falls between 100 and 200, then uses $group with _id: null and a $sum: 1 accumulator - a classic MongoDB pattern for counting all documents that meet a condition into a single result.

Why the distractors are wrong:

• A is wrong because the query doesn't merely fetch posts - it aggregates them; setting _id: null in a $group stage is a grouping mechanism, not a document field update.
• B is wrong because $group with _id: null collapses all matched documents into one group; it doesn't group by individual like counts (101, 102, 103…) or increment anything iteratively.
• D is a partial/misleading description - it conflates $match (filtering) with $group (aggregating) and misrepresents what _id: null does.

Memory tip: In MongoDB aggregation, _id: null inside $group means "treat the entire result set as one group" - pair it with { $sum: 1 } and you're always counting, not fetching or modifying. Think: null = no grouping key = total count.

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