MAPREDUCE Tutorial
From NoSQLZoo
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(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}}
)