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$std DevPop
8.0.1 版中的新增内容。
Amazon DocumentDB 中的$stdDevPop运算符计算数值的总体标准差。作为累加器，它计算聚合管道$group阶段组内文档的总体标准差。作为表达式，它计算数字数组的总体标准差。总体标准差使用 N 作为除数（不是 N-1）。 Non-numeric 值被忽略。如果没有数值，则返回null。如果只有一个数值，则返回0。
参数
示例（MongoDB Shell）
以下示例说明如何使用$stdDevPop运算符计算每个受试者分数的总体标准差。
创建示例文档
db.scores.insertMany([
{ subject: "math", score: 60 },
{ subject: "math", score: 75 },
{ subject: "math", score: 85 },
{ subject: "math", score: 92 },
{ subject: "math", score: 78 },
{ subject: "science", score: 55 },
{ subject: "science", score: 70 },
{ subject: "science", score: 82 },
{ subject: "science", score: 91 },
{ subject: "science", score: 67 }
]);
查询示例
db.scores.aggregate([
{ $group: {
_id: "$subject",
stdDev: { $stdDevPop: "$score" }
}}
]);
输出
[
{ "_id": "math", "stdDev": 10.75174404457249 },
{ "_id": "science", "stdDev": 12.441864811996632 }
]
表达式用法示例（MongoDB Shell）
该$stdDevPop运算符还可用作$project阶段内的表达式来计算数组字段的总体标准差。
创建示例文档
db.experiments.insertMany([
{ _id: 1, measurements: [10, 12, 14, 16, 18] },
{ _id: 2, measurements: [5, 5, 5, 5, 5] },
{ _id: 3, measurements: [2, 4, 6, 8, 10] }
]);
查询示例
db.experiments.aggregate([
{ $project: {
stdDev: { $stdDevPop: "$measurements" }
}}
]);
输出
[
{ "_id": 1, "stdDev": 2.8284271247461903 },
{ "_id": 2, "stdDev": 0 },
{ "_id": 3, "stdDev": 2.8284271247461903 }
]
代码示例
要查看使用$stdDevPop运算符的代码示例，请选择要使用的语言的选项卡。以下示例显示了累加器的用法（输入$group）和表达式用法（中$project）：
- Node.js
-
const { MongoClient } = require('mongodb');
async function example() {
const uri = 'mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false';
const client = new MongoClient(uri);
try {
await client.connect();
const db = client.db('test');
// Accumulator usage: stdDevPop across grouped documents
const scores = db.collection('scores');
const accumulatorResult = await scores.aggregate([
{ $group: {
_id: "$subject",
stdDev: { $stdDevPop: "$score" }
}}
]).toArray();
console.log('Accumulator result:', accumulatorResult);
// Expression usage: stdDevPop of an array field
const experiments = db.collection('experiments');
const expressionResult = await experiments.aggregate([
{ $project: {
stdDev: { $stdDevPop: "$measurements" }
}}
]).toArray();
console.log('Expression result:', expressionResult);
} finally {
await client.close();
}
}
example();
- Python
-
from pymongo import MongoClient
def example():
client = MongoClient('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false')
try:
db = client['test']
# Accumulator usage: stdDevPop across grouped documents
scores = db['scores']
accumulator_result = list(scores.aggregate([
{ '$group': {
'_id': '$subject',
'stdDev': { '$stdDevPop': '$score' }
}}
]))
print('Accumulator result:', accumulator_result)
# Expression usage: stdDevPop of an array field
experiments = db['experiments']
expression_result = list(experiments.aggregate([
{ '$project': {
'stdDev': { '$stdDevPop': '$measurements' }
}}
]))
print('Expression result:', expression_result)
finally:
client.close()
example()
