mongodb 聚合

• 按 name，category，value三个字段进行分组，计算（count）每组的数量

db.getCollection("tools").aggregate(
    [
        { 
            "$group" : {
                "_id" : {
                    "name" : "$name", 
                    "category" : "$category", 
                    "value" : "$value"
                }, 
                "COUNT(*)" : {
                    "$sum" : NumberInt(1)
                }
            }
        }, 
        { 
            "$project" : {
                "name" : "$_id.name", 
                "category" : "$_id.category", 
                "value" : "$_id.value", 
                "COUNT(*)" : "$COUNT(*)", 
                "_id" : NumberInt(0)
            }
        }
    ], 
    { 
        "allowDiskUse" : true
    }
);

• 结果

{ 
    "name" : "盾", 
    "category" : "防具", 
    "value" : NumberInt(11), 
    "COUNT(*)" : NumberInt(1)
}
{ 
    "name" : "披风", 
    "category" : "服装", 
    "value" : NumberInt(9), 
    "COUNT(*)" : NumberInt(1)
}
{ 
    "name" : "剑", 
    "category" : "武器", 
    "value" : NumberInt(11), 
    "COUNT(*)" : NumberInt(1)
}