从符合MongoDb中条件的数组中的文档中提取一个值

db.collection.aggregate([
  {
    $match: {
      scores: {
        $elemMatch: {
          $gte: 80
        }
      }
    }
  }
])

db.collection.aggregate([
  {
    $match: {
      scores: {
        $elemMatch: {
          $gte: 80
        }
      }
    }
  },
  {
    $unwind: "$scores"
  }
])

db.collection.aggregate([
  {
    $match: {
      scores: {
        $elemMatch: {
          $gte: 80
        }
      }
    }
  },
  {
    $unwind: "$scores"
  },
  {
    $project: {
      extractedValue: {
        $cond: {
          if: {
            $gte: ["$scores", 80]
          },
          then: "$scores",
          else: null
        }
      }
    }
  }
])