聚合

没有索引，MongoDB 必须执行集合扫描，即扫描集合中的每个文档。 您不能在_id 字段上删除此索引。

语法

db.collection.aggregate( [ { <stage> }, ... ] )

例子

聚合管道阶段

db.orders.aggregate([
  // Stage 1: Filter pizza order documents by date range
  {
    $match: {
      date: {
        $gte: new ISODate("2020-01-30"),
        $lt: new ISODate("2022-01-30"),
      },
    },
  },

  // Stage 2: Group remaining documents by date and calculate results
  {
    $group: {
      _id: { $dateToString: { format: "%Y-%m-%d", date: "$date" } },
      totalOrderValue: { $sum: { $multiply: ["$price", "$quantity"] } },
      averageOrderQuantity: { $avg: "$quantity" },
    },
  },

  // Stage 3: Sort documents by totalOrderValue in descending order
  {
    $sort: { totalOrderValue: -1 },
  },
]);