$top

运算符 $top 对查询指定的一个或多个字段上的文档进行排序，并返回与筛选条件匹配的第一个文档。 它将排序和选择组合在一个作中，从而有效地查找最高值或最低值，而无需单独的排序阶段。

语法

{
    $top: {
      output: [listOfFields],
      sortBy: {
          < fieldName >: < sortOrder >
      }
  }
}

参数

例子

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
                }
            ]
        }
    ]
}

示例 1：获取每个商店最畅销的类别

若要查找 First Up Consultants 公司内销售最高的类别，请运行查询以检索公司内的商店，按每个类别内总销售额的降序对文档进行排序，并在排序的结果集中返回顶部文档。

db.stores.aggregate([{
    $match: {
        company: {
            $in: ["First Up Consultants"]
        }
    }
}, {
    $group: {
        _id: "$company",
        topSales: {
            $top: {
                output: ["$company", "$sales"],
                sortBy: {
                    "sales.totalSales": -1
                }
            }
        }
    }
}])

此查询返回以下结果：

[
    {
        "_id": "First Up Consultants",
        "topSales": [
            "First Up Consultants",
            {
                "salesByCategory": [
                    {
                        "categoryName": "Towel Sets",
                        "totalSales": 520
                    },
                    {
                        "categoryName": "Bath Accessories",
                        "totalSales": 41710
                    },
                    {
                        "categoryName": "Drapes",
                        "totalSales": 42893
                    },
                    {
                        "categoryName": "Towel Racks",
                        "totalSales": 30773
                    },
                    {
                        "categoryName": "Hybrid Mattresses",
                        "totalSales": 39491
                    },
                    {
                        "categoryName": "Innerspring Mattresses",
                        "totalSales": 6410
                    },
                    {
                        "categoryName": "Bed Frames",
                        "totalSales": 41917
                    },
                    {
                        "categoryName": "Mattress Protectors",
                        "totalSales": 44124
                    },
                    {
                        "categoryName": "Bath Towels",
                        "totalSales": 5671
                    },
                    {
                        "categoryName": "Turkish Towels",
                        "totalSales": 25674
                    }
                ],
                "revenue": 279183
            }
        ]
    }
]

示例 2：按促销类别获取最高折扣

若要提取每个销售类别的最高折扣，请先运行查询以按商店对所有文档进行分组，然后按每个促销事件中的折扣百分比降序对文档进行排序，然后从每个商店的已排序结果集中返回顶部文档。

db.stores.aggregate([{
        $unwind: "$promotionEvents"
    },
    {
        $unwind: "$promotionEvents.discounts"
    },
    {
        $group: {
            _id: "$_id",
            storeName: {
                $first: "$name"
            },
            highestDiscount: {
                $top: {
                    sortBy: {
                        "promotionEvents.discounts.discountPercentage": -1
                    },
                    output: {
                        categoryName: "$promotionEvents.discounts.categoryName",
                        discountPercentage: "$promotionEvents.discounts.discountPercentage",
                        eventName: "$promotionEvents.eventName"
                    }
                }
            }
        }
    }
])

此查询返回的前两个结果为：

[
    {
        "_id": "64ec6589-068a-44a6-be5b-9d37bb0a90f1",
        "storeName": "First Up Consultants | Computer Gallery - West Cathrine",
        "highestDiscount": {
            "categoryName": "Gaming Accessories",
            "discountPercentage": 24,
            "eventName": "Crazy Markdown Madness"
        }
    },
    {
        "_id": "a58d0356-493b-44e6-afab-260aa3296930",
        "storeName": "Fabrikam, Inc. | Outdoor Furniture Nook - West Lexie",
        "highestDiscount": {
            "categoryName": "Fire Pits",
            "discountPercentage": 22,
            "eventName": "Savings Showdown"
        }
    }
]

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