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: JavaScript | 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(_,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(_,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(_,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(_,v){return Array.sum(v)},{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(_,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(_,v){let r={first:'ZZZ',last:'AAA'};for(let i=0;i<v.length;i++){if(r.first>v[i].first){r.first=v[i].first}
if(r.last<v[i].last){r.last=v[i].last}}
return r},{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(_,v){return v},{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(_,v){let r={first:'ZZZ',last:'AAA',count:0};for(let i=0;i<v.length;i++){if(r.first>v[i].first){r.first=v[i].first};if(r.last<v[i].last){r.last=v[i].last};r.count+=1}
return r},{finalize:function(_,v){if(!v.count){v.count=1}
return v},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 p=this.population;switch(!0){case p<1000000:emit("0 TO 1000000",1);break;case p<2000000:emit("1000000 TO 2000000",1);break;case p<3000000:emit("2000000 TO 3000000",1);break;case p<5000000:emit("3000000 TO 5000000",1);break;case p<10000000:emit("5000000 TO 10000000",1);break;case p<15000000:emit("10000000 TO 15000000",1);break
case p>15000000:emit("MORE THAN 15000000",1);break}},function(_,v){return Array.sum(v)},{out:{inline:1}});