$count

$count运算符是聚合管道中使用的累加器，用于计算文档数或分组数。 它通常用于阶段，如$group，$bucket或$bucketAuto$setWindowFields。 此运算符可用于汇总数据或生成特定分组的计数。

语法

{
  $count: "<fieldName>"
}

• <fieldName>：指定将存储计数的输出字段的名称。

参数

参数	DESCRIPTION
<fieldName>	输出文档中将存储计数的字段的名称。

例子

让我们了解使用此架构构建的文档的用法。

{
  "_id": "7954bd5c-9ac2-4c10-bb7a-2b79bd0963c5",
  "name": "Lakeshore Retail | DJ Equipment Stop - Port Cecile",
  "location": {
    "lat": 60.1441,
    "lon": -141.5012
  },
  "staff": {
    "totalStaff": {
      "fullTime": 2,
      "partTime": 0
    }
  },
  "sales": {
    "salesByCategory": [
      {
        "categoryName": "DJ Headphones",
        "totalSales": 35921
      }
    ],
    "fullSales": 3700
  },
  "promotionEvents": [
    {
      "eventName": "Bargain Blitz Days",
      "promotionalDates": {
        "startDate": {
          "Year": 2024,
          "Month": 3,
          "Day": 11
        },
        "endDate": {
          "Year": 2024,
          "Month": 2,
          "Day": 18
        }
      },
      "discounts": [
        {
          "categoryName": "DJ Turntables",
          "discountPercentage": 18
        },
        {
          "categoryName": "DJ Mixers",
          "discountPercentage": 15
        }
      ]
    }
  ],
  "tag": [
    "#ShopLocal",
    "#SeasonalSale",
    "#FreeShipping",
    "#MembershipDeals"
  ],
  "company": "Lakeshore Retail",
  "city": "Port Cecile",
  "lastUpdated": {
    "$date": "2024-12-11T10:21:58.274Z"
  }
}

示例 1：统计文档总数

以下管道对集合中的文档总数进行计数。

db.stores.aggregate([
  {
    $count: "totalDocuments"
  }
])

此查询将返回以下文档。

[
 { totalDocuments: 41501 }
]

---

示例 2：统计按字段分组的文档

以下管道用于 $group 计算每个文档 categoryName 的数量 salesByCategory。

db.stores.aggregate([
  {
    $unwind: "$sales.salesByCategory"
  },
  {
    $group: {
      _id: "$sales.salesByCategory.categoryName",
      totalCount: { $count: {} }
    }
  }
])

此查询将返回以下文档。

[
  { _id: 'Christmas Trees', totalCount: 93 },
  { _id: 'Nuts', totalCount: 83 },
  { _id: 'Camping Tables', totalCount: 130 },
  { _id: 'Music Theory Books', totalCount: 52 },
  { _id: 'Fortified Wine', totalCount: 55 },
  { _id: "Children's Mystery", totalCount: 45 },
  { _id: 'Short Throw Projectors', totalCount: 72 },
  { _id: 'Pliers', totalCount: 56 },
  { _id: 'Bluetooth Headphones', totalCount: 104 },
  { _id: 'Video Storage', totalCount: 80 },
  { _id: 'Cleansers', totalCount: 68 },
  { _id: 'Camera Straps', totalCount: 64 },
  { _id: 'Carry-On Bags', totalCount: 57 },
  { _id: 'Disinfectant Wipes', totalCount: 85 },
  { _id: 'Insignia Smart TVs', totalCount: 81 },
  { _id: 'Toner Refill Kits', totalCount: 38 },
  { _id: 'iPads', totalCount: 51 },
  { _id: 'Memory Foam Mattresses', totalCount: 58 },
  { _id: 'Storage Baskets', totalCount: 68 },
  { _id: 'Body Spray', totalCount: 96 }
]

---

示例 3：计数促销事件

以下管道计数的次数 promotionEvents。

db.stores.aggregate([
  {
    $unwind: "$promotionEvents"
  },
  {
    $count: "totalPromotionEvents"
  }
])

此查询将返回以下文档。

[ 
{ totalPromotionEvents: 145673 } 
]

示例 4：使用$count$setWindowFields

2023 年公司“笔记本电脑”类别的销售计数

db.stores.aggregate([
  { $unwind: "$promotionEvents" },
  { $unwind: "$promotionEvents.discounts" },

  // Filter only for Laptop discounts in 2023
  {
    $match: {
      "promotionEvents.promotionalDates.startDate.Year": 2023,
      "promotionEvents.discounts.categoryName": "Laptops"
    }
  },

  // Add sales count by city using window function
  {
    $setWindowFields: {
      partitionBy: "$company",
      output: {
        salesCount: {
          $count: {},
          window: { documents: ["unbounded", "unbounded"] }
        }
      }
    }
  },

  // Group to return a single result per city
  {
    $group: {
      _id: "$company",
      salesCount: { $first: "$salesCount" }
    }
  }
])

此查询将返回以下文档。

[
  { _id: 'Boulder Innovations', salesCount: 10 },
  { _id: 'VanArsdel, Ltd.', salesCount: 13 },
  { _id: 'Proseware, Inc.', salesCount: 12 },
  { _id: 'Fabrikam, Inc.', salesCount: 11 },
  { _id: 'Contoso, Ltd.', salesCount: 13 },
  { _id: 'Fourth Coffee', salesCount: 8 },
  { _id: 'Trey Research', salesCount: 14 },
  { _id: 'Adatum Corporation', salesCount: 12 },
  { _id: 'Relecloud', salesCount: 16 },
  { _id: 'Lakeshore Retail', salesCount: 13 },
  { _id: 'Northwind Traders', salesCount: 5 },
  { _id: 'First Up Consultants', salesCount: 4 },
  { _id: 'Wide World Importers', salesCount: 7 },
  { _id: 'Tailwind Traders', salesCount: 12 }
]

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