计算不同的子文档字段并输出为命名关键字 [英] Count Distinct Sub document field and Output as Named Keys

问题描述

在MongoDB中，如果我有一个如下所示的集合，

{
    "auctionId" : 22,
    "startDt" : "2017-08-28T06:00:00.000Z",
    "endDt" : "2017-09-04T06:00:00.000Z",
    "status" : "Open",
    "pickupDt" : "2017-09-07T06:00:00.000Z",
    "itmLst" : 
    [ 
        {
            "itemId" : 1,
            "location" : "Open",
            "currentBid" : 13.0,
            "highBidder" : 1897,
            "bidCnt" : 4,
            "catgegory" : "ANTIQUES"
             ...

我该如何使用聚合函数查询它以获取AuctionId = 22的不同类别的计数，并按itmLst.category分组，因此它将返回如下所示的结果集:

{
    "ANTIQUES": 56,
    "TOOLS": 89,
    "JEWLRY": 45,
    ...
}

解决方案

我找到了我要找的答案.该查询:

返回:

谢谢尼尔，尽管我指出了正确的方向.

In MongoDB, If I had a collection that looked like the following,

{
    "auctionId" : 22,
    "startDt" : "2017-08-28T06:00:00.000Z",
    "endDt" : "2017-09-04T06:00:00.000Z",
    "status" : "Open",
    "pickupDt" : "2017-09-07T06:00:00.000Z",
    "itmLst" : 
    [ 
        {
            "itemId" : 1,
            "location" : "Open",
            "currentBid" : 13.0,
            "highBidder" : 1897,
            "bidCnt" : 4,
            "catgegory" : "ANTIQUES"
             ...

How could i query it with an aggregate function to retrieve a count of the distinct categories for auctionId = 22, grouping by itmLst.category, so it would return a result set that would look like this:

{
    "ANTIQUES": 56,
    "TOOLS": 89,
    "JEWLRY": 45,
    ...
}

解决方案

I have found the answer i was looking for. This query:

db.auctions.aggregate([
   { "$match": { "auctionId": 22 } },
   { $unwind:'$itmLst' }, 
   { $group:{_id:'$itmLst.catgegory', freq:{$sum:1}} },
   { $sort:{ "_id": 1 } }
]);

returns:

{
    "_id" : "ANTIQUES",
    "freq" : 9.0
}
{
    "_id" : "APPLIANCES",
    "freq" : 1.0
}
{
    "_id" : "ARTS CRAFTS",
    "freq" : 9.0
}
{
    "_id" : "BOOKS MAGAZINES",
    "freq" : 6.0
}

Thank you Neil for pointing me in the right direction though.

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