mongo简单的聚合使用

mongo 聚合索引

• 根据某字段进行分组统计

  • 数据源结构

    { "_id" : ObjectId("5fc7694da343baa23c0b2d75"), "id" : 0, "age" : 13, "name" : "name869", "created" : ISODate("2020-12-02T10:15:41.615Z") }
    { "_id" : ObjectId("5fc7694da343baa23c0b2d76"), "id" : 1, "age" : 101, "name" : "name471", "created" : ISODate("2020-12-02T10:15:41.616Z") }
    { "_id" : ObjectId("5fc7694da343baa23c0b2d77"), "id" : 2, "age" : 29, "name" : "name115", "created" : ISODate("2020-12-02T10:15:41.616Z") }
    { "_id" : ObjectId("5fc7694da343baa23c0b2d78"), "id" : 3, "age" : 73, "name" : "name382", "created" : ISODate("2020-12-02T10:15:41.616Z") }
    { "_id" : ObjectId("5fc7694da343baa23c0b2d79"), "id" : 4, "age" : 1, "name" : "name258", "created" : ISODate("2020-12-02T10:15:41.617Z") }
    { "_id" : ObjectId("5fc7694da343baa23c0b2d7a"), "id" : 5, "age" : 117, "name" : "name586", "created" : ISODate("2020-12-02T10:15:41.617Z") }
    { "_id" : ObjectId("5fc7694da343baa23c0b2d7b"), "id" : 6, "age" : 29, "name" : "name15", "created" : ISODate("2020-12-02T10:15:41.618Z") }
    { "_id" : ObjectId("5fc7694da343baa23c0b2d7c"), "id" : 7, "age" : 55, "name" : "name774", "created" : ISODate("2020-12-02T10:15:41.618Z") }
    { "_id" : ObjectId("5fc7694da343baa23c0b2d7d"), "id" : 8, "age" : 14, "name" : "name138", "created" : ISODate("2020-12-02T10:15:41.619Z") }
    { "_id" : ObjectId("5fc7694da343baa23c0b2d7e"), "id" : 9, "age" : 63, "name" : "name920", "created" : ISODate("2020-12-02T10:15:41.619Z") }
    

  • 假设根据 name 进行聚合统计

    1. 查询映射到指定的字段 使用关键字 $project

       {"$project":{"name":1}}
       

    2. 对 name字段进行分组，并计算总出现的次数 count

       {"$group":{"_id":"$name","count":{"$sum":1}}}
       

    3. 对已经计算好的次数进行排序 使用关键字 $sort

       {"$sort":{"count":-1}} # -1 代表降序  1 代表升序
       

    4. 取出前10条记录

       {"$limit":10}
       

    5. 将以上的步骤都需要放在聚合查询的函数里 关键字 aggregate

       db.blog.aggregate(
           {"$project":{"name":1}},
           {"$group":{"_id":"$name","count":{"$sum":1}}},
           {"$sort":{"count":-1}},
           {"$limit":10}
       )