如何在MongoDB中找到一个字段的平均值？

Question

我正在尝试在 js 文件中查找书籍的平均价格。

到目前为止，我已经计算出了总共 4 本书的数量（查询如下所示）

db.bookshop.aggregate([ {"$unwind":"$book"},
                          {"$project":{"price":"$book.book","_id":0}},
                          {"$count":"average price of books"} ]).pretty();

此查询的输出：

{ "average price of books" : 4 }

这是我试图找出所有不起作用的书籍的总价格。我已经这样做了，因为我假设找到平均值是总和除以计数。

db.bookshhop.aggregate([ {$group: {_id: null,"TotalAmount": 
{$sum: "$book.price"}}}] );

我在查找所有图书价格的平均价格时遇到问题。我怎样才能做到这一点？谢谢

这些是书的js记录：

 db.bookshop.insert( {
    "_id":"185.3.16",
    "book": {
        "callnum":"185.3.16",
        "isbn":"1-292-06118-9",
        "title":"Database Systems",
        "authors":[
            {
                "fname":"Thomas",
                "lname":"Connolly"},
            { 
                "fname":"Carolyn",
                "lname":"Begg"}
        ],
        "publisher":"Pearson Pty Ltd",
        "year":2015,
        "price":136.99,
        "topic":"Computer Science",
        "description":"This is the 6th edition. You can register online to access the examples",
        "keywords":["Database", "XML", "Distributed"]
        }
});

db.bookshop.insert( {
    "_id":"163.24.12",
    "book": {
        "callnum":"163.24.12",
        "isbn":"1-123-456-810",
        "title":"Core Java",
        "authors":[
            {
                "fname":"Horstmann",
                "lname":"Cornell"}
        ],
        "publisher":"PH Pty Ltd",
        "year":2012,
        "price":142.90,
        "topic":"Computer Science",
        "description":"It covers JAVA programming and JAVA script",
        "keywords":["JAVA", "XML", "Script"]
    }
});

db.bookshop.insert( {
    "_id":"123.45.67",
    "book": {
        "callnum":"123.45.67",
        "isbn":"1-123-456-789",
        "title":"Algorithms",
        "authors":[
            {
                "fname":"James",
                "lname":"Bond"},
            {
                "fname":"Harry",
                "lname":"Potter"},
            {
                "fname":"William",
                "lname":"Stallings"}
        ],
        "publisher":"Pearson Pty Ltd",
        "year":2013,
        "price":65.85,
        "topic":"Computer Science",
        "description":"It contains algorithms and their applications. You can download examples from the website"
    }
});
db.bookshop.insert( {
    "_id":"134.41.33",
    "book": {
        "callnum":"134.41.33",
        "isbn":"1-213-431-770",
        "title":"C++ Programming",
        "authors":[
            {
                "fname":"Larry",
                "lname":"Peterson"}
        ],
        "publisher":"Pearson Pty Ltd",
        "year":2010,
        "price":74.90,
        "topic":"Computer Science",
        "description":"C++ programming and its applications",
        "keywords":["C++", "Class", "Overloading", "Inheritance"]
    }
});

Answer 1

您可以使用 $avg 获得平均价格。

文档：https://docs.mongodb.com/manual/reference/operator/aggregation/avg

db.bookshop.aggregate([
  {
    $group: {
      _id: null,
      AvgPrice: {
        $avg: "$book.price"
      }
    }
  }
]);

这将给出以下结果：

[
  {
    "AvgPrice": 105.16,
    "_id": null
  }
]

这是一个可以测试的游乐场。 https://mongoplayground.net/p/csfgxFBbYfx

Answer 2

db.collection.aggregate( [{ $group: {  _id: "Books Price", "average": {    "$avg": "$book.price"  }} }] )