Question

SELECT name, COUNT(*)
FROM (
    SELECT string_agg(name ORDER BY name) name
    FROM mytable
    GROUP BY startts, endts
    ORDER BY name
) as names
GROUP BY name
ORDER BY name

我正在数据库中查询两次以查找文档总数。如何通过对数据库的单个查询来做到这一点？

Answer 1

//you can use aggregation pipeline and $facet to combine two DB requests into one
//Actual Code output from windows Mongoshell/Command Line Interface(CLI)
> print("mongoDB",db.version())
mongoDB 4.2.6
> db.groups1.aggregate([
... {$facet:{
...   "docCount":[
...     {$group:{
...       _id:null,
...       count:{$sum:1}
...       }
...      }
...     ],
...   "allDocs":[
...      {$match:{}}
...      ]
...  }
...  }
... ]).pretty();
{
        "docCount" : [
                {
                        "_id" : null,
                        "count" : 4
                }
        ],
        "allDocs" : [
                {
                        "_id" : ObjectId("5f817d142689512ef0bd3bd7"),
                        "uid" : 1,
                        "group_id" : "g1"
                },
                {
                        "_id" : ObjectId("5f817d142689512ef0bd3bd8"),
                        "uid" : 1,
                        "group_id" : "g2"
                },
                {
                        "_id" : ObjectId("5f817d142689512ef0bd3bd9"),
                        "uid" : 2,
                        "group_id" : "g2"
                },
                {
                        "_id" : ObjectId("5f817d142689512ef0bd3bd1"),
                        "uid" : 2,
                        "group_id" : "g3"
                }
        ]
}
>

Answer 2

使用$facet聚合阶段将多个嵌套管道组合到您的主管道中。

这是上述想法的有效代码-

const countPipeline = [
    {
        $match: query
    },
    {
        $group: {
            _id: null,
            count: {
                $sum: 1
            }
        }
    },
    {
        $project: {
            _id: 0,
            count: '$count'
        }
    }
];

const postPipeline = [
    {
        $match: query
    },
    {
        $skip: startIndex
    },
    {
        $limit: noOfRecordsPerPage
    },
    {
        $sort: {
            createdAt: -1
        }
    }
];

const pipeline = [
    {
        $facet: {
            posts: postPipeline,
            count: countPipeline
        }
    }
];

const results = await Posts.aggreagate(pipeline).exec();

在单个查询中计算猫鼬中的文档

2 个答案: