問題描述

有誰知道如何在 PouchDB 數(shù)據(jù)庫上創(chuàng)建聚合函數(shù)，例如 avg、sum、max 和 min.我創(chuàng)建了一個簡單的應(yīng)用程序來測試 PouchDB.我仍然不知道如何運行這些命令.提前致謝.

Does anyone know how to create aggregate functions, for example avg, sum, max and min on PouchDB database. I created a simple application to test the PouchDB. I'm still not figured out how to run these commands. Thanks in advance.

例如.您如何獲得數(shù)字"字段的最高、最低或平均值?

For example. How do you get the highest, lowest or average for the "number" field?

我的主要 Ionic 2 組件

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);

推薦答案

您可以使用 map/reduce 函數(shù) 來自 PouchDB 的 db.query() 方法 以獲得平均值、總和、最大或任何其他類型的聚合文檔.

You can use the map/reduce functions of the db.query() method from PouchDB to get the average, sum, largest or any other kind of aggregation of the docs.

我創(chuàng)建了一個 演示 JSBin fiddle 和一個正在運行的示例.我將函數(shù)的解釋直接添加到代碼中(如下)作為注釋，因為我認(rèn)為它會更簡單.

I have created a demo JSBin fiddle with a running example. I added the explanation of the functions directly into the code (below) as comments, as I thought it'd be simpler.

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

    // http://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:這只是一種方法.還有其他幾種可能性(不將 ID 作為鍵發(fā)出，使用組和不同的 reduce 函數(shù)，使用內(nèi)置的 reduce 函數(shù)，例如 _sum，...)，我只是認(rèn)為一般來說這是更簡單的選擇.

Note: This is just one way to do it. There are several other possibilities (not emitting IDs as keys, using groups and different reduce functions, using built-in reduce functions, such as _sum, ...), I just thought this was the simpler alternative generally speaking.

這篇關(guān)于如何在 PouchDB 上模擬聚合函數(shù) avg、sum、max、min 和 count?的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網(wǎng)！