Question

Hello all i have different document containing "category" field i have to get result from aggregation query to find number of document in each category and find result in array which sould contain {_id and num } field;


    {
       "_id": NumberInt(23),
       "title": "WiredTiger T-shirt",
       "slogan": "Unleash the tiger",
       "description": "Crafted from ultra-soft combed cotton, this essential t-shirt features sporty contrast tipping and MongoDB's signature leaf.",
       "stars": NumberInt(0),
       "category": "Apparel",
       "img_url": "/img/products/wt-shirt.jpg",
       "price": 22 
    }


    {
       "_id": NumberInt(22),
       "title": "Water Bottle",
       "slogan": "Glass water bottle",
       "description": "High quality glass bottle provides a healthier way to drink. Silicone sleeve provides a good grip, a see-through window, and protects the glass vesse [...]",
       "stars": NumberInt(0),
       "category": "Kitchen",
       "img_url": "/img/products/water-bottle.jpg",
       "price": 23 
    }

我有很多像这样的文件。我使用了以下查询： -

db.collectionname.find（{类别： “类别名称”}，{_ ID：1}）计数（）; 但在这个阶段我只会得到一个类别的数据。

Answer 1

您必须为此查询使用组。对于组，您必须在mongodb中使用aggreagetion。以下将做你想要的：

db.collection.aggregate([{$group: {_id: "$category", num: {$sum: 1}}}]);

mongodb文档也有一个例子： https://docs.mongodb.org/manual/reference/command/count/

mongodb聚合查询无法创建

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