运算符 $bottom 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最后一个文档。
语法
运算符的 $isArray 语法如下所示：
{
$bottom: {
output: [listOfFields],
sortBy: {
<fieldName>: < sortOrder >
}
}
}
参数
| 参数 | DESCRIPTION |
|---|---|
listOfFields |
要从结果集中的最后一个文档返回的字段列表 |
fieldName |
用于对结果集进行排序的字段 |
sortOrder |
1 或 -1。 1 表示按字段值的升序排序，而 -1 表示按字段值的降序排序 |
示例：
请考虑存储集合中的此示例文档。
{
"_id": "0fcc0bf0-ed18-4ab8-b558-9848e18058f4",
"name": "First Up Consultants | Beverage Shop - Satterfieldmouth",
"location": {
"lat": -89.2384,
"lon": -46.4012
},
"staff": {
"totalStaff": {
"fullTime": 8,
"partTime": 20
}
},
"sales": {
"totalSales": 75670,
"salesByCategory": [
{
"categoryName": "Wine Accessories",
"totalSales": 34440
},
{
"categoryName": "Bitters",
"totalSales": 39496
},
{
"categoryName": "Rum",
"totalSales": 1734
}
]
},
"promotionEvents": [
{
"eventName": "Unbeatable Bargain Bash",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 6,
"Day": 23
},
"endDate": {
"Year": 2024,
"Month": 7,
"Day": 2
}
},
"discounts": [
{
"categoryName": "Whiskey",
"discountPercentage": 7
},
{
"categoryName": "Bitters",
"discountPercentage": 15
},
{
"categoryName": "Brandy",
"discountPercentage": 8
},
{
"categoryName": "Sports Drinks",
"discountPercentage": 22
},
{
"categoryName": "Vodka",
"discountPercentage": 19
}
]
},
{
"eventName": "Steal of a Deal Days",
"promotionalDates": {
"startDate": {
"Year": 2024,
"Month": 9,
"Day": 21
},
"endDate": {
"Year": 2024,
"Month": 9,
"Day": 29
}
},
"discounts": [
{
"categoryName": "Organic Wine",
"discountPercentage": 19
},
{
"categoryName": "White Wine",
"discountPercentage": 20
},
{
"categoryName": "Sparkling Wine",
"discountPercentage": 19
},
{
"categoryName": "Whiskey",
"discountPercentage": 17
},
{
"categoryName": "Vodka",
"discountPercentage": 23
}
]
}
]
}
示例 1：查找总销售额最低的商店
假设我们想要确定销售额最低的 First Up 顾问公司内的商店，运行查询以检索 First Up Consultants 公司内的文档，按总销售额的降序对文档进行排序，并在排序的结果集中返回最后一个文档。
db.stores.aggregate([{
"$match": {
"company": {
"$in": ["First Up Consultants"]
}
}
}, {
"$group": {
"_id": "$company",
"bottomSales": {
"$bottom": {
"output": ["$company", "$sales"],
"sortBy": {
"sales.revenue": -1
}
}
}
}
}])
此查询返回以下结果：
[
{
"_id": "First Up Consultants",
"bottomSales": [
"First Up Consultants",
{
"totalSales": 119,
"salesByCategory": [
{
"categoryName": "Skirts",
"totalSales": 109
}
]
}
]
}]
示例 2：查找销售额最低的每个商店的类别
若要查找每个商店销售额最低的类别，请运行查询以检索具有多个销售类别的商店，按每个商店内总销售额的降序对类别进行排序，并返回排序结果集中的最后一个文档。
db.stores.aggregate([{
$unwind: "$sales.salesByCategory"
},
{
$match: {
"sales.salesByCategory.totalSales": {
$exists: true
}
}
},
{
$group: {
_id: "$_id",
storeName: {
$first: "$name"
},
lowestCategory: {
$bottom: {
sortBy: {
"sales.salesByCategory.totalSales": 1
},
output: {
categoryName: "$sales.salesByCategory.categoryName",
totalSales: "$sales.salesByCategory.totalSales"
}
}
}
}
}
])
此查询返回的前两个结果为：
[
{
"_id": "b1d86d1f-8705-4157-b64c-a9eda0df4921",
"storeName": "VanArsdel, Ltd. | Baby Products Haven - West Kingfort",
"lowestCategory": { "categoryName": "Baby Monitors", "totalSales": 49585 }
},
{
"_id": "22e6367e-8341-415f-9795-118d2b522abf",
"storeName": "Adatum Corporation | Outdoor Furniture Mart - Port Simone",
"lowestCategory": { "categoryName": "Outdoor Benches", "totalSales": 4976 }
}
]