mongodb集合中不存在月份如何返回0

db.collection.aggregate([
  {
    $match: {
      month: "指定月份"
    }
  }
])

db.collection.aggregate([
  {
    $match: {
      month: "指定月份"
    }
  },
  {
    $group: {
      _id: null,
      count: {
        $sum: 1
      }
    }
  }
])

db.collection.aggregate([
  {
    $match: {
      month: "指定月份"
    }
  },
  {
    $group: {
      _id: null,
      count: {
        $sum: 1
      }
    }
  },
  {
    $project: {
      _id: 0,
      count: {
        $cond: {
          if: { $eq: ["$count", 0] },
          then: 0,
          else: "$count"
        }
      }
    }
  }
])