MAPREDUCE Tutorial

MapReduce the basics

This tutorial introduces the MapReduce command.

MapReduce examples are available.

The MapReduce function allows to functions to be applied over a collection. The two functions map and reduce are important because:

Together they can solve a huge range of common database queries.
They can be distributed effectively; you can share the work load among a number of processors and get your answer faster.

It is recommended to have some familiarity with JavaScript before attempting the following questions: | MDN

Map

The map function can be used to filter and transform the data:

Use a map function to get the names of countries in Europe.

db.world.mapReduce(
  function(){
    if (this.continent === "Europe"){
      emit(this.name, null);
    }
  },
  function(k, v){ 
    return v; 
  },
  {out:{inline:1}}  
)

db.world.mapReduce(
  function(){if (this.continent=="Europe"){emit(this.name,null);}},
  function(k, v){ return v; },
  {out:{inline:1}}  
)

Use the previous answer to find the population of the world to the nearest million.

Use the JavaScript round function : Math.round(population/1000000)*1000000

db.world.mapReduce(
  function(){
    emit(null, null);
  },
  function(k, v){ 
    return null; 
  },
  {out:{inline:1}}  
)

db.world.mapReduce(
    function(){emit('World Population in Millions', this.population)},
    function(key, values){
      return Math.round(Array.sum(values)/1000000)*1000000;
    },
    {out: {inline :1}}
)

For each letter, determine how many country names begin with that letter.

db.world.mapReduce(
  function(){
    emit(null, null);
  },
  function(k, v){ 
    return null; 
  },
  {out:{inline:1}}  
)

db.world.mapReduce(
    function(){ emit((this.name).substring(0,1), 1)},
    function(key, values){
       return Array.sum(values);
    },
    {out: {inline:1}}
)

Show the number of countries in each continent.

db.world.mapReduce(
  function(){
    emit(null, null);
  },
  function(k, v){ 
    return null; 
  },
  {out:{inline:1}}  
)

db.world.mapReduce(
    function(){emit(this.continent, 1)},
    function(key, values){
      return Array.sum(values);
    },
    {out: {inline :1}}
)

Show the name and area of the smallest 3 countries by area. Ignore records where the area is 0 or null.

db.world.mapReduce(
  function(){
    emit(null, null);
  },
  function(k, v){ 
    return null; 
  },
  {out:{inline:1}}  
)

db.world.mapReduce(
        function(){emit(this.name, this.area);}, 
        function(k, v){ return v;},
        {
            limit: 3,
            out: {inline:1},
            query: {"$and": [{"area": {"$ne": null}}, {"area": {"$ne": 0}}]},
            sort: {"area": 1}
        }
)

For each continent, find the first and last country alphabetically.

db.world.mapReduce(
  function(){
    emit(null, null);
  },
  function(k, v){ 
    return null; 
  },
  {out:{inline:1}}  
)

db.world.mapReduce(
    function(){emit(this.continent, {first:this.name, last:this.name});},
    function(key, values){
      let ret = {first:'ZZZ', last:'AAA'};
      for(let i=0; i < values.length; i++){
        if (ret.first > values[i].first) { ret.first = values[i].first; }
        if (ret.last < values[i].last) { ret.last = values[i].last; }
      }
      return ret;
    },
    {out: {inline:1}}
)

Return country names or capital cities that start with a letter 'M' as keys, use null as the value.

db.world.mapReduce(
  function(){
    emit(null, null);
  },
  function(k, v){ 
    return null; 
  },
  {out:{inline:1}}  
)

db.world.mapReduce(
    function(){ 
        if((this.name).startsWith('M')) {emit(this.name,null); }
        if((this.capital).startsWith('M')) {emit(this.capital,null); }
    },
    function(key, values){
       return values;
    },    
    {out: {inline :1}}
)

For each letter in the alphabet, find the amount of cities and the cities that come first and last alphabetically. e.g.

{
	"_id" : "B",
	"value" : {
		"first" : "Baghdad",
		"last" : "Bujumbura",
		"count" : 25
	}
},

db.world.mapReduce(
  function(){
    emit(null, null);
  },
  function(k, v){ 
    return null; 
  },
  {out:{inline:1}}  
)

db.world.mapReduce(
    function(){ if(this.capital)emit((this.capital).substring(0,1), {first:this.capital, last:this.capital})},
    function(key, values){
      let ret = {first:'ZZZ',last:'AAA', count:0};
      for(let i=0; i < values.length; i++){
        if (ret.first>values[i].first) { ret.first = values[i].first };
        if (ret.last<values[i].last) { ret.last = values[i].last };
        ret.count += 1;
      }
      return ret;
    },
    {
        finalize: function(key, val){
            if(!val.count){
               val.count = 1;
            }
            return val;
        },
        out: {inline: 1}
    }
)

Show country count for countries in the following ranges:

 0 to 1000000
 1000000 to 2000000
 2000000 to 3000000
 3000000 to 5000000
 5000000 to 10000000
 10000000 to 15000000
 More than 15000000

db.world.mapReduce(
  function(){
    emit(null, null);
  },
  function(k, v){ 
    return null; 
  },
  {out:{inline:1}}  
)

db.world.mapReduce(
    function(){
        var pop = this.population;
        switch(true){
            case pop<1000000:
               emit("0 TO 1000000", 1);
               break;
            case pop<2000000:
               emit("1000000 TO 2000000", 1);
               break;
            case pop<3000000:
               emit("2000000 TO 3000000", 1);
               break;
            case pop<5000000:
               emit("3000000 TO 5000000", 1);
               break;
            case pop<10000000:
               emit("5000000 TO 10000000", 1);
               break;
            case pop<15000000:
               emit("10000000 TO 15000000", 1);
               break
            case pop>15000000:
               emit("MORE THAN 15000000", 1);
               break;
        }
    },
    function(key, values){ return Array.sum(values); },
    {out: {inline: 1}} 
)