javascript – 如何模拟PouchDB上的聚合函数avg,sum,max,min和count?
作者:互联网
有谁知道如何在PouchDB数据库上创建聚合函数,例如avg,sum,max和min.我创建了一个简单的应用程序来测试PouchDB.我还没弄明白如何运行这些命令.提前致谢.
例如.您如何获得“数字”字段的最高,最低或平均值?
My main Ionic 2 component
import {Component} from '@angular/core';
import {Platform, ionicBootstrap} from 'ionic-angular';
import {StatusBar} from 'ionic-native';
import {HomePage} from './pages/home/home';
declare var require: any;
var pouch = require('pouchdb');
var pouchFind = require('pouchdb-find');
@Component({
template: '<ion-nav [root]="rootPage"></ion-nav>'
})
export class MyApp {
rootPage: any = HomePage;
db: any;
value: any;
constructor(platform: Platform) {
platform.ready().then(() => {
StatusBar.styleDefault();
});
pouch.plugin(pouchFind);
this.db = new pouch('friendsdb');
let docs = [
{
'_id': '1',
'number': 10,
'values': '1, 2, 3',
'loto': 'fooloto'
},
{
'_id': '2',
'number': 12,
'values': '4, 7, 9',
'loto': 'barloto'
},
{
'_id': '3',
'number': 13,
'values': '9, 4, 5',
'loto': 'fooloto'
}
];
this.db.bulkDocs(docs).then(function (result) {
console.log(result);
}).catch(function (err) {
console.log(err);
});
}
}
ionicBootstrap(MyApp);
解决方法:
您可以使用map / reduce函数of the db.query()
method from PouchDB来获取文档的平均值,总和,最大值或任何其他类型的聚合.
我创建了一个demo JSBin fiddle with a running example.我将函数的解释直接添加到代码(下面)作为注释,因为我认为它更简单.
var db = new PouchDB('friendsdb');
var docs = [
{'_id': '1', 'number': 10, 'values': '1, 2, 3', 'loto': 'fooloto'},
{'_id': '2', 'number': 12, 'values': '4, 7, 9', 'loto': 'barloto'},
{'_id': '3', 'number': 13, 'values': '9, 4, 5', 'loto': 'fooloto'}
];
db.bulkDocs(docs).then(function(result) {
querySum();
queryLargest();
querySmallest();
queryAverage();
}).catch(function(err) {
console.log(err);
});
function querySum() {
function map(doc) {
// the function emit(key, value) takes two arguments
// the key (first) arguments will be sent as an array to the reduce() function as KEYS
// the value (second) arguments will be sent as an array to the reduce() function as VALUES
emit(doc._id, doc.number);
}
function reduce(keys, values, rereduce) {
// keys:
// here the keys arg will be an array containing everything that was emitted as key in the map function...
// ...plus the ID of each doc (that is included automatically by PouchDB/CouchDB).
// So each element of the keys array will be an array of [keySentToTheEmitFunction, _idOfTheDoc]
//
// values
// will be an array of the values emitted as value
console.info('keys ', JSON.stringify(keys));
console.info('values ', JSON.stringify(values));
// check for more info: http://couchdb.readthedocs.io/en/latest/couchapp/views/intro.html
// So, since we want the sum, we can just sum all items of the values array
// (there are several ways to sum an array, I'm just using vanilla for to keep it simple)
var i = 0, totalSum = 0;
for(; i < values.length; i++){
totalSum += values[i];
}
return totalSum;
}
db.query({map: map, reduce: reduce}, function(err, response) {
console.log('sum is ' + response.rows[0].value);
});
}
function queryLargest() {
function map(doc) {
emit(doc._id, doc.number);
}
function reduce(keys, values, rereduce) {
// everything same as before (see querySum() above)
// so, this time we want the larger element of the values array
// https://stackoverflow.com/a/1379560/1850609
return Math.max.apply(Math, values);
}
db.query({map: map, reduce: reduce}, function(err, response) {
console.log('largest is ' + response.rows[0].value);
});
}
function querySmallest() {
function map(doc) {
emit(doc._id, doc.number);
}
function reduce(keys, values, rereduce) {
// all the same... now the looking for the min
return Math.min.apply(Math, values);
}
db.query({map: map, reduce: reduce}, function(err, response) {
console.log('smallest is ' + response.rows[0].value);
});
}
function queryAverage() {
function map(doc) {
emit(doc._id, doc.number);
}
function reduce(keys, values, rereduce) {
// now simply calculating the average
var i = 0, totalSum = 0;
for(; i < values.length; i++){
totalSum += values[i];
}
return totalSum/values.length;
}
db.query({map: map, reduce: reduce}, function(err, response) {
console.log('average is ' + response.rows[0].value);
});
}
注意:这只是一种方法.还有其他几种可能性(不使用ID作为键,使用组和不同的reduce函数,使用内置的reduce函数,例如_sum,…),我只是认为这是一般说来的更简单的替代方法.
标签:indexeddb,pouchdb,javascript,nosql,database 来源: https://codeday.me/bug/20191008/1871999.html