$lastN 累加器运算符返回一组文档中指定表达式的最后 N 个值。 需要从排序的集合中检索多个最终值而不仅仅是最后那个值时，它很有用。
Syntax
{
$group: {
_id: < expression > ,
< field >: {
$lastN: {
n: < number >,
input: < expression >
}
}
}
}
参数
| 参数 | Description |
|---|---|
n |
要返回的值的数目。 必须是正整数。 |
input |
表达式，用于指定要返回最后 N 次出现的字段或值。 |
例子
请考虑商店集合中的这个示例文档。
{
"_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：使用 $lastN 作为累积器按商店查找最后两个促销事件
若要检索每个商店的最后两个促销事件，请运行一个查询，按开始日期的升序对促销事件进行排序，按存储对排序的事件进行分组，并返回每个商店中的最后两个事件。
db.stores.aggregate([{
$unwind: "$promotionEvents"
},
{
$sort: {
"promotionEvents.promotionalDates.startDate.Year": 1,
"promotionEvents.promotionalDates.startDate.Month": 1,
"promotionEvents.promotionalDates.startDate.Day": 1
}
},
{
$group: {
_id: "$_id",
storeName: {
$last: "$name"
},
lastTwoPromotions: {
$lastN: {
input: "$promotionEvents.eventName",
n: 2
}
},
lastTwoPromotionDates: {
$lastN: {
input: "$promotionEvents.promotionalDates.startDate",
n: 2
}
}
}
}
])
此查询返回的前两个结果为：
[
{
"_id": "e28fff9b-a8fb-4ac9-bb37-dea60d2a7d32",
"storeName": "Lakeshore Retail | Outdoor Furniture Collection - Erdmanside",
"lastTwoPromotions": [
"Big Bargain Bash",
"Spectacular Savings Showcase"
],
"lastTwoPromotionDates": [
{
"Year": 2024,
"Month": 9,
"Day": 21
},
{
"Year": 2024,
"Month": 6,
"Day": 23
}
]
},
{
"_id": "1bec7539-dc75-4f7e-b4e8-afdf8ff2f234",
"storeName": "Adatum Corporation | Health Food Market - East Karina",
"lastTwoPromotions": [
"Price Slash Spectacular",
"Spectacular Savings Showcase"
],
"lastTwoPromotionDates": [
{
"Year": 2024,
"Month": 9,
"Day": 21
},
{
"Year": 2024,
"Month": 6,
"Day": 23
}
]
}
]
示例 2：使用$lastN作为累积器查找三大销售类别的销售
若要检索每个商店销售类别最高的销售类别，请运行查询以销售量升序对销售类别进行排序，按商店对排序结果进行分组，并返回每个商店的最后三个类别。
db.stores.aggregate([{
$unwind: "$sales.salesByCategory"
},
{
$sort: {
"sales.salesByCategory.totalSales": 1
}
},
{
$group: {
_id: "$_id",
storeName: {
$last: "$name"
},
top3Categories: {
$lastN: {
input: "$sales.salesByCategory.categoryName",
n: 3
}
},
top3SalesAmounts: {
$lastN: {
input: "$sales.salesByCategory.totalSales",
n: 3
}
}
}
}
])
此查询返回的前两个结果为：
[
{
"_id": "22e6367e-8341-415f-9795-118d2b522abf",
"storeName": "Adatum Corporation | Outdoor Furniture Mart - Port Simone",
"top3Categories": [
"Outdoor Benches"
],
"top3SalesAmounts": [
4976
]
},
{
"_id": "a00a3ccd-49a2-4e43-b0d9-e56b96113ed0",
"storeName": "Wide World Importers | Smart Home Deals - Marcuschester",
"top3Categories": [
"Smart Thermostats",
"Smart Plugs"
],
"top3SalesAmounts": [
38696,
633
]
}
]
示例 3：使用 $lastN 运算符作为数组表达式获取最后两个升级事件
该示例演示了操作员使用情况，用于从商店中查找最近两个促销事件。
db.stores.aggregate([
{ $match: {"_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74"} },
{
$project: {
name: 1,
lastTwoPromotions: {
$lastN: {
input: "$promotionEvents",
n: 2
}
}
}
}
])
此查询返回以下结果。
[
{
"_id": "40d6f4d7-50cd-4929-9a07-0a7a133c2e74",
"name": "Proseware, Inc. | Home Entertainment Hub - East Linwoodbury",
"lastTwoPromotions": [
{
"eventName": "Grand Deal Days",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 6,
"Day": 23
},
"endDate": {
"Year": 2024,
"Month": 6,
"Day": 30
}
},
"discounts": [
{
"categoryName": "Remote Controls",
"discountPercentage": 7
},
{
"categoryName": "Televisions",
"discountPercentage": 11
},
{
"categoryName": "Business Projectors",
"discountPercentage": 13
},
{
"categoryName": "Laser Projectors",
"discountPercentage": 6
},
{
"categoryName": "Projectors",
"discountPercentage": 6
},
{
"categoryName": "Projector Screens",
"discountPercentage": 24
}
]
},
{
"eventName": "Major Bargain Bash",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 9,
"Day": 21
},
"endDate": {
"Year": 2024,
"Month": 9,
"Day": 30
}
},
"discounts": [
{
"categoryName": "Sound Bars",
"discountPercentage": 9
},
{
"categoryName": "VR Games",
"discountPercentage": 7
},
{
"categoryName": "Xbox Games",
"discountPercentage": 25
},
{
"categoryName": "Projector Accessories",
"discountPercentage": 18
},
{
"categoryName": "Mobile Games",
"discountPercentage": 8
},
{
"categoryName": "Projector Cases",
"discountPercentage": 22
}
]
}
]
}
]
相关内容
- 查看用于 从 MongoDB 迁移到 Azure DocumentDB 的选项。
- 详细了解 与 MongoDB 的功能兼容性。