$bucket and $bucketAuto

$group by category is a discrete key. `$bucket` groups a numeric (or date) range. Price bands, age brackets, histograms. The alternative is $group plus a pile of $cond — that is the expressions module, and it is worse to read.

db.products.aggregate([
  {
    $bucket: {
      groupBy: "$price",
      boundaries: [0, 1000, 5000, 20000],
      default: "other",
      output: {
        count: { $sum: 1 },
        avgPrice: { $avg: "$price" }
      }
    }
  }
])

{ _id: 0, count: 8, avgPrice: 420 }
{ _id: 1000, count: 15, avgPrice: 2800 }
{ _id: 5000, count: 10, avgPrice: 8900 }
{ _id: "other", count: 2, avgPrice: 25000 }

boundaries must be sorted, same type, at least two values. Documents outside the range go to default; omit default and those documents are dropped. $bucketAuto picks boundaries for you: { $bucketAuto: { groupBy: "$price", buckets: 4 } } — even-ish counts, not pretty round numbers. Use $bucket when the bands are a product decision (₹0–1k / 1k–5k). Use $bucketAuto when you want N groups and do not care where the cuts fall.