mongodb中两个分组值的总和？

现在，我如何计算我帐户中的余额金额，如（C）之和-（D）之和=650？

对于这种聚合操作类型，十进制操作符将派上用场。您可以使用它来计算给定

Tran_Type

值的总和，因此您可以将此逻辑注入管道阶段，在该阶段您将对所有文档进行分组（即，按空键分组）。您还需要使用一个操作符来计算与算术操作符的差异

下面的示例将演示此概念。运行管道操作以获得所需的结果：

填充测试文档：

Trans_Id      Account_Id     Amount   Tran_Type
   1             HA1001       1000       C
   2             HA1001         50       D
   3             HA1001        100       C
   4             HA1001        400       D

聚合管道：

db.test.insert([
    {
        "Trans_Id": 1,
        "Account_Id": "HA1001",
        "Amount": 1000,
        "Tran_Type": "C"
    },
    {
        "Trans_Id": 2,
        "Account_Id": "HA1001",
        "Amount": 50,
        "Tran_Type": "D"
    },
    {
        "Trans_Id": 3,
        "Account_Id": "HA1001",
        "Amount": 100,
        "Tran_Type": "C"
    },
    {
        "Trans_Id": 4,
        "Account_Id": "HA1001",
        "Amount": 400,
        "Tran_Type": "D"
    }
])

db.test.aggregate([
    {
        "$group": {
            "_id": null,
            "total_C": {
                "$sum": {
                    "$cond": [ 
                        { "$eq": [ "$Tran_Type", "C" ] }, 
                        "$Amount", 0 
                    ]
                }
            },
            "total_D": {
                "$sum": {
                    "$cond": [ 
                        { "$eq": [ "$Tran_Type", "D" ] }, 
                        "$Amount", 0 
                    ]
                }
            }
        }
    },
    {
        "$project": {
            "balance": {
                "$subtract": ["$total_C", "$total_D"]
            }, "_id": 0
        }
    }
])

样本输出：

db.test.insert([
    {
        "Trans_Id": 1,
        "Account_Id": "HA1001",
        "Amount": 1000,
        "Tran_Type": "C"
    },
    {
        "Trans_Id": 2,
        "Account_Id": "HA1001",
        "Amount": 50,
        "Tran_Type": "D"
    },
    {
        "Trans_Id": 3,
        "Account_Id": "HA1001",
        "Amount": 100,
        "Tran_Type": "C"
    },
    {
        "Trans_Id": 4,
        "Account_Id": "HA1001",
        "Amount": 400,
        "Tran_Type": "D"
    }
])

db.test.aggregate([
    {
        "$group": {
            "_id": null,
            "total_C": {
                "$sum": {
                    "$cond": [ 
                        { "$eq": [ "$Tran_Type", "C" ] }, 
                        "$Amount", 0 
                    ]
                }
            },
            "total_D": {
                "$sum": {
                    "$cond": [ 
                        { "$eq": [ "$Tran_Type", "D" ] }, 
                        "$Amount", 0 
                    ]
                }
            }
        }
    },
    {
        "$project": {
            "balance": {
                "$subtract": ["$total_C", "$total_D"]
            }, "_id": 0
        }
    }
])

---

如果我还有一个列属于类别，例如“Cat1”、“Cat2”，还需要一个帮助。现在我如何才能获得余额类别？HassanAhamed看起来原来的问题已经解决了，不是吗？至于“下一个”问题，那将是一个，考虑单独发布。