Question

下面的查询给出了每年模型的总和。我需要找到最大模型的年份。（数据是制造商中的另一个数组）

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Answer 1

按年度分组
计算当年制作的型号总数
根据型号总和进行排序
将查询限制为1

现在你将获得最大型号的年份。

所以你的Query（Java实现）将如下所示：

     // Group by Year
     DBObject groupFields = new BasicDBObject( "_id", "$_id.year");

     //Count total no of models made in that year
     groupFields.put("sum", new BasicDBObject( "$sum", "$manufacturers.data.modelName"));

     //Group the fields
     DBObject group = new BasicDBObject("$group", groupFields);

     //Sort in descending order inorder to get maximum models 
     DBObject sortFields = new BasicDBObject("sum", -1);
     DBObject sort = new BasicDBObject("$sort", sortFields );

     //Limit to one since you need the year when maximum models were made
     DBObject limit = new BasicDBObject("$limit", 1 );
     AggregationOutput output = myColl.aggregate(group,sort,limit);

希望这有帮助。

编辑输入：

db.structure.aggregate([
{$unwind : "$manufacturers"},
    {$group: {
        _id: {
            day:"$_id.year"
        },
        count: { $sum: "$manufacturers.data.modelName" }
    }},
    {$sort: {count : -1}},
    //Limit by 1 to get year when maximum models where made
    {$limit : 1},
    {$out : "out"}
]);

mongodb中总和的最大值

1 个答案: