MongoDB中的条件$ sum

Question

我在mongodb中的集合类似于SQL中的下表:

情感(公司,感悟)

现在,我需要执行这样的查询:

SELECT
  Company, 
  SUM(CASE WHEN Sentiment >0 THEN Sentiment ELSE 0 END) AS SumPosSenti, 
  SUM(CASE WHEN Sentiment <0 THEN Sentiment ELSE 0 END) AS SumNegSenti
FROM Sentiments
GROUP BY Company

我该怎么做才能在Mongo中编写这个查询？我被困在以下查询中:

db.Sentiments.aggregate(
{ $project: {_id:0, Company:1, Sentiment: 1} },
{ $group: {_id: "$Company", SumPosSenti: {$sum: ? }, SumNegSenti: {$sum: ? } } }
);

Answer 1

正如Sammaye建议的那样,您需要使用$cond聚合投影运算符来执行此操作:

db.Sentiments.aggregate(
    { $project: {
        _id: 0,
        Company: 1,
        PosSentiment: {$cond: [{$gt: ['$Sentiment', 0]}, '$Sentiment', 0]},
        NegSentiment: {$cond: [{$lt: ['$Sentiment', 0]}, '$Sentiment', 0]}
    }},
    { $group: {
        _id: "$Company",
        SumPosSentiment: {$sum: '$PosSentiment'},
        SumNegSentiment: {$sum: '$NegSentiment'}
    }});

Answer 2

从版本3.4开始,我们可以使用$switch允许在$group阶段中进行逻辑条件处理的运算符.当然我们仍然需要使用$sum累加器来返回总和.

db.Sentiments.aggregate(
    [
        { "$group": { 
            "_id": "$Company",  
            "SumPosSenti": { 
                "$sum": { 
                    "$switch": { 
                        "branches": [ 
                            { 
                                "case": { "$gt": [ "$Sentiment", 0 ] }, 
                                "then": "$Sentiment"
                            }
                        ], 
                        "default": 0 
                    }
                }
            }, 
            "SumNegSenti": {
                "$sum": { 
                    "$switch": { 
                        "branches": [ 
                            { 
                                "case": { "$lt": [ "$Sentiment", 0 ] }, 
                                "then": "$Sentiment"
                            }
                        ], 
                        "default": 0 
                    } 
                }
            }
        }}
    ]
)

如果您尚未迁移您mongod到3.4或更高版本,那么请注意,$project在这个阶段,答案是多余的,因为$cond运营商返回一个数值,这意味着你可以$group你的文件和应用$sum的$cond表达.

这将改善您的应用程序的性能,尤其是对于大型集合.

db.Sentiments.aggregate(
    [
        { '$group': {
            '_id': '$Company',
            'PosSentiment': { 
                '$sum': {
                    '$cond': [
                        { '$gt': ['$Sentiment', 0]}, 
                        '$Sentiment', 
                        0
                    ]
                }
            },
            'NegSentiment': { 
                '$sum': {
                    '$cond': [
                        { '$lt': ['$Sentiment', 0]}, 
                        '$Sentiment', 
                        0
                    ]
                }
            }
        }}
    ]
)

考虑一个带有以下文档的集合Sentiments:

{ "Company": "a", "Sentiment" : 2 }
{ "Company": "a", "Sentiment" : 3 }
{ "Company": "a", "Sentiment" : -1 }
{ "Company": "a", "Sentiment" : -5 }

聚合查询产生:

{ "_id" : "a", "SumPosSenti" : 5, "SumNegSenti" : -6 }

Answer 3

解释上面使用数组语法的片段：

PosSentiment: {$cond: [{$gt: ['$Sentiment', 0]}, '$Sentiment', 0]}

等于：

PosSentiment: {$cond: { if: {$gt: ['$Sentiment', 0]}, then: '$Sentiment', else: 0} }

数组语法将长语法总结为 { $cond: [if, then, else] }