$subtract

运算符 $subtract 用于减去两个数字并返回结果。

{
  $subtract: [ <expression 1>, <expression 2> ]
}

请考虑存储集合中的此示例文档。

{
    "_id": "0fcc0bf0-ed18-4ab8-b558-9848e18058f4",
    "name": "First Up Consultants | Beverage Shop - Satterfieldmouth",
    "location": {
        "lat": -89.2384,
        "lon": -46.4012
    },
    "staff": {
        "totalStaff": {
            "fullTime": 8,
            "partTime": 20
        }
    },
    "sales": {
        "totalSales": 75670,
        "salesByCategory": [
            {
                "categoryName": "Wine Accessories",
                "totalSales": 34440
            },
            {
                "categoryName": "Bitters",
                "totalSales": 39496
            },
            {
                "categoryName": "Rum",
                "totalSales": 1734
            }
        ]
    },
    "promotionEvents": [
        {
            "eventName": "Unbeatable Bargain Bash",
            "promotionalDates": {
                "startDate": {
                    "Year": 2024,
                    "Month": 6,
                    "Day": 23
                },
                "endDate": {
                    "Year": 2024,
                    "Month": 7,
                    "Day": 2
                }
            },
            "discounts": [
                {
                    "categoryName": "Whiskey",
                    "discountPercentage": 7
                },
                {
                    "categoryName": "Bitters",
                    "discountPercentage": 15
                },
                {
                    "categoryName": "Brandy",
                    "discountPercentage": 8
                },
                {
                    "categoryName": "Sports Drinks",
                    "discountPercentage": 22
                },
                {
                    "categoryName": "Vodka",
                    "discountPercentage": 19
                }
            ]
        },
        {
            "eventName": "Steal of a Deal Days",
            "promotionalDates": {
                "startDate": {
                    "Year": 2024,
                    "Month": 9,
                    "Day": 21
                },
                "endDate": {
                    "Year": 2024,
                    "Month": 9,
                    "Day": 29
                }
            },
            "discounts": [
                {
                    "categoryName": "Organic Wine",
                    "discountPercentage": 19
                },
                {
                    "categoryName": "White Wine",
                    "discountPercentage": 20
                },
                {
                    "categoryName": "Sparkling Wine",
                    "discountPercentage": 19
                },
                {
                    "categoryName": "Whiskey",
                    "discountPercentage": 17
                },
                {
                    "categoryName": "Vodka",
                    "discountPercentage": 23
                }
            ]
        }
    ]
}

若要计算“首次启动顾问”公司内商店的兼职和全职员工绝对差异，请首先运行查询，按公司名称筛选商店。 然后，使用 $diff 运算符和 $abs 运算符计算每个商店全职员工和兼职员工之间的绝对差异。

db.stores.aggregate([{
    "$match": {
        "company": {
            "$in": ["First Up Consultants"]
        }
    }
}, {
    "$project": {
        "name": 1,
        "staff": 1,
        "staffCountDiff": {
            $abs: {
                "$subtract": ["$staff.employeeCount.fullTime", "$staff.employeeCount.partTime"]
            }
        }
    }
}])

此查询返回的前三个结果为：

[
    {
        "_id": "62438f5f-0c56-4a21-8c6c-6bfa479494ad",
        "name": "First Up Consultants | Plumbing Supply Shoppe - New Ubaldofort",
        "staff": {
            "employeeCount": {
                "fullTime": 20,
                "partTime": 18
            }
        },
        "staffCountDiff": 2
    },
    {
        "_id": "bfb213fa-8db8-419f-8e5b-e7096120bad2",
        "name": "First Up Consultants | Beauty Product Shop - Hansenton",
        "staff": {
            "employeeCount": {
                "fullTime": 18,
                "partTime": 10
            }
        },
        "staffCountDiff": 8
    },
    {
        "_id": "14ab145b-0819-4d22-9e02-9ae0725fcda9",
        "name": "First Up Consultants | Flooring Haven - Otisville",
        "staff": {
            "employeeCount": {
                "fullTime": 19,
                "partTime": 10
            }
        },
        "staffCountDiff": 9
    }
]

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