在 MongoDB 中组合组 - 聚合

Question

我在MySQL中有这段代码对应一个简单的汽车数据库

SELECT
"Cost" AS "Metric",
AVG(car_cost) As "Avg",
MIN(car_cost) As "Min",
MAX(car_cost) As "Max",
"Dollars" As "Unit"
from db.car_data

union SELECT
'Mileage',
AVG(car_mileage),
MIN(car_mileage),
MAX(car_mileage),
'Miles'
from db.car_data

那个returns这个table

Metric	Avg	Min	Max
Cost	2345	5674	53456
Mileage	49023	53	178003

但是，当在 MongoDB 中重新创建它时，我运行变成了一个问题

 db.car_data.aggregate([
  {"$group": 
    {
      _id: null,
      Avg: {
        $avg: { "$car_cost" }
      },
      Min: {
        $min: { "$car_cost" }
      },
      Max: {
        $max: { "$car_cost" }
      }
    }
  },
  {"$group": 
    {
      _id: null,
      Avg: {
        $avg: { car_mileage }
      },
      Min: {
        $min: { car_mileage }
      },
      Max: {
        $max: { car_mileage }
      }
    }
  }
]);

实际输出

[
  {
    "_id": null,
    "Avg": null,
    "Min": null,
    "Max": null
  }
]

预期输出

[
  {
    "_id": null,
    "Avg": 2345,
    "Min": 5674,
    "Max": 53456
  },
  {
    "_id": null,
    "Avg": 49023,
    "Min": 53,
    "Max": 178003
  }
]

我是在做错什么，还是有更好的方法通过 Mongo 中的聚合来做到这一点？他们每个人都在 Mongo 独自工作...

Answer 1

查询

你在 1 个数据库中有 1 个集合，你想要 6 个累加器
您可以将它们分成 3 个一组，甚至分成 2 个文档，以获得与结果

*我们有 $facet $unionWith 等，但您不需要这些，无需进行多次聚合，只需 1 个即可获得所需的一切。

Test code here

aggregate(
[ {
  "$group" : {
    "_id" : null,
    "avg_cost" : {
      "$avg" : "$car_cost"
    },
    "min_cost" : {
      "$min" : "$car_cost"
    },
    "max_cost" : {
      "$max" : "$car_cost"
    },
    "avg_mileage" : {
      "$avg" : "$car_mileage"
    },
    "min_mileage" : {
      "$min" : "$car_mileage"
    },
    "max_mileage" : {
      "$max" : "$car_mileage"
    }
  }
}, {
  "$set" : {
    "array" : [ {
      "metric" : "cost",
      "avg" : "$avg_cost",
      "min" : "$min_cost",
      "max" : "$max_cost"
    }, {
      "metric" : "mileage",
      "avg" : "$avg_mileage",
      "min" : "$min_mileage",
      "max" : "$max_mileage"
    } ]
  }
}, {
  "$unwind" : {
    "path" : "$array"
  }
}, {
  "$replaceRoot" : {
    "newRoot" : "$array"
  }
} ]
)

在 MongoDB 中组合组 - 聚合

Combining groups in MongoDB - Aggregation

aggregate-functions

mongodb

aggregation-framework