该 $sample 阶段用于聚合管道，从集合中随机选择指定数量的文档。 此命令 $sample 在测试、数据分析和生成用于机器学习的随机数据子集期间非常有用。
语法
{
"$sample": { "size": <number> }
}
参数
| 参数 | DESCRIPTION |
|---|---|
size |
要从集合中随机选择的文档数 |
例子
请查看 StoreData 数据库中 stores 集合的这个示例文档。
{
"_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 - 随机选择五个文档并投影相应的文档 ID
db.stores.aggregate([{"$sample": {"size": 5}}, {"$project": {"_id": 1}}])
此查询返回以下结果：
[
{ "_id": "f7ae8b40-0c66-4e80-9261-ab31bbabffb4" },
{ "_id": "25350272-6797-4f98-91f8-fe79084755c7" },
{ "_id": "c7fd1d22-1a29-4cb0-9155-1ad71d600c2b" },
{ "_id": "e602b444-9519-42e3-a2e1-b5a3da5f6e64" },
{ "_id": "189c239a-edca-434b-baae-aada3a27a2c5" }
]