MongoDB如何投影count前5项 剩余项求和为Other并计算占比

直接使用MongoDB聚合管道即可在数据库层完成所有计算，不需要额外业务代码处理，你的集合已经按count字段降序排序，直接运行如下聚合语句即可：

// 替换成你实际的集合名
db.team_collection.aggregate([
  {
    $facet: {
      // 取排名前5的项
      topTeams: [
        { $limit: 5 },
        { $project: { _id: 0, name: "$_id", count: 1 } }
      ],
      // 剩余项求和归为Other
      otherTeams: [
        { $skip: 5 },
        { $group: { _id: null, count: { $sum: "$count" } } },
        { $project: { _id: 0, name: { $literal: "Other" }, count: 1 } }
      ]
    }
  },
  {
    $project: {
      mergedList: { $concatArrays: ["$topTeams", "$otherTeams"] },
      totalCount: { $sum: { $concatArrays: ["$topTeams.count", "$otherTeams.count"] } }
    }
  },
  // 计算每一项的占比
  {
    $project: {
      finalResult: {
        $map: {
          input: "$mergedList",
          as: "item",
          in: {
            name: "$$item.name",
            count: "$$item.count",
            percent: { $round: [{ $multiply: [{ $divide: ["$$item.count", "$totalCount"] }, 100] }, 0] }
          }
        }
      }
    }
  },
  { $replaceRoot: { newRoot: "$finalResult" } }
])

• 如果你需要调整百分比的小数精度，修改$round操作符的第二个参数即可，传入2就会保留两位小数
• 你提供的示例中百分比为示意值，上述语句会按照全量总count计算真实占比，针对你给出的测试数据，运行后返回结果结构如下：

[
  {"name":"Team 1", "count":1200, "percent":17},
  {"name":"Team 2", "count":1170, "percent":17},
  {"name":"Team 3", "count":1006, "percent":14},
  {"name":"Team 4", "count":932, "percent":13},
  {"name":"Team 5", "count":931, "percent":13},
  {"name":"Other", "count":1794, "percent":26}
]

内容的提问来源于stack exchange，提问作者TheStranger