mongo 数据库平均差异

Question

我正在为一些作业中的 MongoDB 查询而苦苦挣扎。我是 MongoDb 的新手，到目前为止只了解基础知识。这是问题：

Suppose that we have full-time results data since 2001-08-01 for a variety of football competitions in a mongo database collection results which has the following fields kick_off_date, competition_id, home_team_id, away_team_id, home_score, away_score.

If the competition_id of the English Premiership is 1 and the team_id of Everton is 5, write the mongo query that would return the average difference in goals scored and goals conceded by Everton since 2005-01-01 in >English Premiership games where they were playing away.

到目前为止我有这个：

db.results.find( { competition_id,: 1, away_team_id: 5, kick_off_date: { $gte : new ISODate("2005-1-1T00:00:00Z") }  } )

我认为这是自 2005-01-01 以来埃弗顿在英超联赛中的相关数据。但是除了使用电子表格之外，我不知道如何返回进球数和失球数的平均差异。

谁能给我指出正确的方向？

Answer 1

首先要注意：像你一样，我是 mongoDB 的新手，所以尽管下面给出的答案似乎有效，但可能有更简洁/更高效的方法来实现相同的结果。

这是我构建的示例数据集：

/* 0 */
{
    "_id" : ObjectId("54d62ce0e11e084bc1366195"),
    "kick_off_date" : ISODate("2005-01-16T10:35:54.985Z"),
    "competition_id" : 1,
    "home_team_id" : 1,
    "away_team_id" : 5,
    "home_score" : 1,
    "away_score" : 3
}

/* 1 */
{
    "_id" : ObjectId("54d62cece11e084bc1366196"),
    "kick_off_date" : ISODate("2005-02-16T10:35:54.985Z"),
    "competition_id" : 1,
    "home_team_id" : 2,
    "away_team_id" : 5,
    "home_score" : 3,
    "away_score" : 1
}

/* 2 */
{
    "_id" : ObjectId("54d62cfde11e084bc1366197"),
    "kick_off_date" : ISODate("2005-03-16T10:35:54.985Z"),
    "competition_id" : 1,
    "home_team_id" : 3,
    "away_team_id" : 5,
    "home_score" : 5,
    "away_score" : 0
}

/* 3 */
{
    "_id" : ObjectId("54d62d0ce11e084bc1366198"),
    "kick_off_date" : ISODate("2005-04-16T10:35:54.985Z"),
    "competition_id" : 1,
    "home_team_id" : 4,
    "away_team_id" : 5,
    "home_score" : 0,
    "away_score" : 5
}

从这个数据可以看出，净胜球是：

    "_id" : ObjectId("54d62ce0e11e084bc1366195"),
    "difference" : -2

    "_id" : ObjectId("54d62cece11e084bc1366196"),
    "difference" : 2

    "_id" : ObjectId("54d62cfde11e084bc1366197"),
    "difference" : 5

    "_id" : ObjectId("54d62d0ce11e084bc1366198"),
    "difference" : -5

因为里面的负差，把这些值加起来就是0，这是没有用的。因此查询必须考虑到这一点并将负数视为正数以给出总数 14。平均差异将是：(14/4) = 3.5

下面是执行所有这些操作的聚合查询：

db.full_time_results.aggregate(
    {$match:
        {
            competition_id: 1,
            away_team_id: 5,
            kick_off_date: { $gte : ISODate("2005-01-01T00:00:00Z") }
        }
    },
    {$project:
        {
            away_team_id:1,
            difference: {$subtract:["$home_score","$away_score"]}
        }
    },
    {$group:
        {
            _id:"$away_team_id",
            avg_difference: {$avg:
                {$cond: 
                    { 
                        if: { $lt: [ "$difference", 0 ] }, 
                        then: {$multiply:["$difference", -1]}, 
                        else: "$difference" 
                    }
                }
             }
        }
    }
)

最后，结果：

{
    "_id" : 5,
    "avg_difference" : 3.5
}

mongo 数据库平均差异

mongo DB average difference

mongodb

average

difference