常见关系代数运算包括：选择、投影、并、交、差以及自然连接操作等，都可以十分容易利用MapReduce框架进行并行化计算
| NAME | SEX | AGE |
| 小明 | 男 | 25 |
| 小红 | 女 | 18 |
| 小张 | 男 | 22 |
| 小米 | 女 | 23 |
| 小丽 | 女 | 21 |
| 小王 | 男 | 19 |
| 小美 | 女 | 25 |
| 小朱 | 女 | 26 |
选择操作
将关系R的数据存储在relationR文件，然后移入HDFS下的data文件夹，如代码1-1
代码1-1
root@lejian:/data# cat relationR 小明 男 25 小红 女 18 小张 男 22 小米 女 23 小丽 女 21 小王 男 19 小美 女 25 小朱 女 26 root@lejian:/data# hadoop fs -put selection /data root@lejian:/data# hadoop fs -ls -R /data -rw-r--r-- 1 root supergroup 112 2017-01-07 15:03 /data/relationR
对于关系R的应用条件C，选择性别为女的数据，只需在Map阶段对每个输入的记录进行判断，将满足条件的数据输出即可，输出键值为(key,null)。Reduce阶段无需做额外的工作
代码1-2
<?xml version="1.0"?> <configuration> <property> <name>sex</name> <value>女</value> </property> </configuration>
代码1-3
package com.hadoop.mapreduce;
public class Person {
private String name;
private String sex;
private int age;
public Person(String line) {
super();
String[] lines = line.split(" ");
this.name = lines[0];
this.sex = lines[1];
this.age = Integer.parseInt(lines[2]);
}
public String getName() {
return name;
}
public String getSex() {
return sex;
}
public int getAge() {
return age;
}
public String getVal(String col) {
if ("name".equals(col)) {
return name;
}
if ("sex".equals(col)) {
return sex;
}
return age + "";
}
@Override
public String toString() {
return name + " " + sex + " " + age;
}
}
代码1-4
package com.hadoop.mapreduce;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
public class SelectionMap extends Mapper<LongWritable, Text, Text, NullWritable> {
private String sex = "";
private Text val = new Text();
protected void setup(Context context) throws java.io.IOException, InterruptedException {
Configuration conf = context.getConfiguration();
sex = conf.get("sex");
};
protected void map(LongWritable key, Text value, Context context) throws java.io.IOException, InterruptedException {
Person person = new Person(value.toString());
if (sex.equals(person.getVal("sex"))) {
val.set(person.toString());
context.write(val, NullWritable.get());
}
};
}
代码1-5
package com.hadoop.mapreduce;
import java.io.IOException;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
public class Selection {
public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
if (args == null || args.length != 2) {
throw new RuntimeException("请输入输入路径、输出路径");
}
Configuration conf = new Configuration();
conf.addResource("conf.xml");
Job job = Job.getInstance(conf);
job.setJobName("Selection");
job.setMapperClass(SelectionMap.class);
job.setOutputKeyClass(Text.class);
job.setOutputValueClass(NullWritable.class);
FileInputFormat.addInputPaths(job, args[0]);
FileOutputFormat.setOutputPath(job, new Path(args[1]));
System.exit(job.waitForCompletion(true) ? 0 : 1);
}
}
运行代码1-5，运行结果如代码1-6
代码1-6
root@lejian:/data# hadoop jar selection.jar com.hadoop.mapreduce.Selection /data /output ………… root@lejian:/data# hadoop fs -ls -R /output -rw-r--r-- 1 root supergroup 0 2017-01-07 15:05 /output/_SUCCESS -rw-r--r-- 1 root supergroup 70 2017-01-07 15:05 /output/part-r-00000 root@lejian:/data# hadoop fs -cat /output/part-r-00000 小丽 女 21 小朱 女 26 小米 女 23 小红 女 18 小美 女 25
投影操作
例如在关系R上应用投影操作获得属性AGE的所有值，我们只需要在Map阶段将每条记录的AGE属性和NullWritable输出，而Reduce端仅获取key即可，注意，此时投影操作具有去重功能
代码1-7
package com.hadoop.mapreduce;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
public class ProjectionMap extends Mapper<LongWritable, Text, IntWritable, NullWritable> {
private IntWritable age = new IntWritable();
protected void map(LongWritable key, Text value, Context context) throws java.io.IOException, InterruptedException {
Person person = new Person(value.toString());
age.set(person.getAge());
context.write(age, NullWritable.get());
};
}
代码1-8
package com.hadoop.mapreduce;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.mapreduce.Reducer;
public class ProjectionReduce extends Reducer<IntWritable, NullWritable, IntWritable, NullWritable> {
protected void reduce(IntWritable key, Iterable<NullWritable> values, Context context) throws java.io.IOException, InterruptedException {
context.write(key, NullWritable.get());
};
}
代码1-9
package com.hadoop.mapreduce;
import java.io.IOException;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
public class Projection {
public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
if (args == null || args.length != 2) {
throw new RuntimeException("请输入输入路径、输出路径");
}
Configuration conf = new Configuration();
Job job = Job.getInstance(conf);
job.setJobName("Projection");
job.setMapperClass(ProjectionMap.class);
job.setReducerClass(ProjectionReduce.class);
job.setOutputKeyClass(IntWritable.class);
job.setOutputValueClass(NullWritable.class);
FileInputFormat.addInputPaths(job, args[0]);
FileOutputFormat.setOutputPath(job, new Path(args[1]));
System.exit(job.waitForCompletion(true) ? 0 : 1);
}
}
运行代码1-9，运行结果如代码1-10
代码1-10
root@lejian:/data# hadoop jar projection.jar com.hadoop.mapreduce.Projection /data /output ………… root@lejian:/data# hadoop fs -ls -R /output -rw-r--r-- 1 root supergroup 0 2017-01-07 15:52 /output/_SUCCESS -rw-r--r-- 1 root supergroup 21 2017-01-07 15:52 /output/part-r-00000 root@lejian:/data# hadoop fs -cat /output/part-r-00000 18 19 21 22 23 25 26
交运算
如果有一个关系A和关系B为同一个模式，希望得到关系A和关系B的交集，那么在Map阶段对于A和B中的每一条记录r输出(r,1)，在Reduce阶段汇总计数，如果计数为2，则将该条记录输出。依旧以Person类为例，这里把Person作为主键，为了使得关系A和关系B相同的Person发送到同一个Reduce节点进行计算，需要对原先代码1-3的Person类进行修改，如代码1-11，MapReduce默认会先调用对象的compareTo方法进行对象间的比较，如果对象相等，再比较其hashCode，如果hashCode相等，则认为这两个对象为同一个对象
修改代码1-3的Person类为代码1-11
代码1-11
package com.hadoop.mapreduce;
import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import org.apache.hadoop.io.WritableComparable;
public class Person implements WritableComparable<Person> {
private String name;
private String sex;
private int age;
public Person() {
super();
// TODO Auto-generated constructor stub
}
public Person(String line) {
super();
String[] lines = line.split(" ");
this.name = lines[0];
this.sex = lines[1];
this.age = Integer.parseInt(lines[2]);
}
public String getName() {
return name;
}
public String getSex() {
return sex;
}
public int getAge() {
return age;
}
public String getVal(String col) {
if ("name".equals(col)) {
return name;
}
if ("sex".equals(col)) {
return sex;
}
return age + "";
}
@Override
public String toString() {
return name + " " + sex + " " + age;
}
@Override
public int hashCode() {
int res = 20;
res = name.hashCode() + 10 * res;
res = sex.hashCode() + 10 * res;
res = age + 10 * res;
return res;
}
@Override
public void write(DataOutput out) throws IOException {
out.writeUTF(name);
out.writeUTF(sex);
out.writeInt(age);
}
@Override
public void readFields(DataInput in) throws IOException {
name = in.readUTF();
sex = in.readUTF();
age = in.readInt();
}
@Override
public int compareTo(Person o) {
// TODO Auto-generated method stub
if (hashCode() > o.hashCode()) {
return 1;
}
if (hashCode() < o.hashCode()) {
return -1;
}
return 0;
}
public static void main(String[] args) {
System.out.println(new Person("Lily female 22").hashCode());
}
}
将关系A和关系B移入HDFS下的data文件夹，如代码1-12
root@lejian:/data# cat relationA Tom male 21 Amy female 19 Daivd male 16 Lily female 22 Lucy female 20 John male 19 Rose female 19 Jojo female 26 root@lejian:/data# cat relationB Daivd male 16 Jack male 15 Lily female 22 Lucy female 20 Tom male 25 root@lejian:/data# hadoop fs -put relation* /data root@lejian:/data# hadoop fs -ls -R /data -rw-r--r-- 1 root supergroup 113 2017-01-07 20:48 /data/relationA -rw-r--r-- 1 root supergroup 69 2017-01-07 20:48 /data/relationB
代码1-13
package com.hadoop.mapreduce;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
public class IntersectionMap extends Mapper<LongWritable, Text, Person, IntWritable> {
private static final IntWritable ONE = new IntWritable(1);
protected void map(LongWritable key, Text value, Context context) throws java.io.IOException, InterruptedException {
Person person = new Person(value.toString());
context.write(person, ONE);
};
}
代码1-14
package com.hadoop.mapreduce;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.mapreduce.Reducer;
public class IntersectionReduce extends Reducer<Person, IntWritable, Person, NullWritable> {
protected void reduce(Person key, Iterable<IntWritable> values, Context context) throws java.io.IOException, InterruptedException {
int count = 0;
for (IntWritable val : values) {
count += val.get();
}
if (count == 2) {
context.write(key, NullWritable.get());
}
};
}
代码1-15
package com.hadoop.mapreduce;
import java.io.IOException;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
public class Intersection {
public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
if (args == null || args.length != 2) {
throw new RuntimeException("请输入输入路径、输出路径");
}
Configuration conf = new Configuration();
Job job = Job.getInstance(conf);
job.setJobName("Intersection");
job.setJarByClass(Intersection.class);
job.setMapperClass(IntersectionMap.class);
job.setMapOutputKeyClass(Person.class);
job.setMapOutputValueClass(IntWritable.class);
job.setReducerClass(IntersectionReduce.class);
job.setOutputKeyClass(Person.class);
job.setOutputValueClass(NullWritable.class);
FileInputFormat.addInputPaths(job, args[0]);
FileOutputFormat.setOutputPath(job, new Path(args[1]));
System.exit(job.waitForCompletion(true) ? 0 : 1);
}
}
运行代码1-15，运行结果如代码1-16
代码1-16
root@lejian:/data# hadoop jar intersection.jar com.hadoop.mapreduce.Intersection /data /output ………… root@lejian:/data# hadoop fs -ls -R /output -rw-r--r-- 1 root supergroup 0 2017-01-07 20:30 /output/_SUCCESS -rw-r--r-- 1 root supergroup 44 2017-01-07 20:30 /output/part-r-00000 root@lejian:/data# hadoop fs -cat /output/part-r-00000 Daivd male 12 Lily female 22 Lucy female 20
差运算
计算关系A-关系B的差集，即找出在关系A中存在而在关系B中不存在的记录，在Map阶段，对于关系A和关系B中每一条记录r输出键值对(r,A)，(r,B)，在Reduce阶段检查每一条记录r和其对应的关系名称，只有关系名称只存在A，才输出记录
先显示HDFS中data文件夹下得relationA和relationB的文件内容，如代码1-17
代码1-17
root@lejian:/data# hadoop fs -ls -R /data -rw-r--r-- 1 root supergroup 113 2017-01-07 20:48 /data/relationA -rw-r--r-- 1 root supergroup 69 2017-01-07 20:48 /data/relationB root@lejian:/data# hadoop fs -cat /data/relationA Tom male 21 Amy female 19 Daivd male 16 Lily female 22 Lucy female 20 John male 19 Rose female 19 Jojo female 26 root@lejian:/data# hadoop fs -cat /data/relationB Daivd male 16 Jack male 15 Lily female 22 Lucy female 20 Tom male 25
代码1-18
package com.hadoop.mapreduce;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.lib.input.FileSplit;
public class DifferenceMap extends Mapper<LongWritable, Text, Person, Text> {
private Text relationName = new Text();
protected void setup(Context context) throws java.io.IOException, InterruptedException {
FileSplit fileSplit = (FileSplit) context.getInputSplit();
relationName.set(fileSplit.getPath().getName());
};
protected void map(LongWritable key, Text value, Context context) throws java.io.IOException, InterruptedException {
Person person = new Person(value.toString());
context.write(person, relationName);
};
}
代码1-19
package com.hadoop.mapreduce;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;
public class DifferenceReduce extends Reducer<Person, Text, Person, NullWritable> {
private String remove = "";
protected void setup(Context context) throws java.io.IOException, InterruptedException {
Configuration conf = context.getConfiguration();
remove = conf.get("remove");
};
protected void reduce(Person key, Iterable<Text> values, Context context) throws java.io.IOException, InterruptedException {
for (Text val : values) {
if (remove.equals(val.toString())) {
return;
}
}
context.write(key, NullWritable.get());
};
}
代码1-20
package com.hadoop.mapreduce;
import java.io.IOException;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
public class Difference {
public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
if (args == null || args.length != 3) {
throw new RuntimeException("请输入输入路径、输出路径和被减集合");
}
Configuration conf = new Configuration();
conf.set("remove", args[2]);
Job job = Job.getInstance(conf);
job.setJobName("Difference");
job.setJarByClass(Difference.class);
job.setMapperClass(DifferenceMap.class);
job.setMapOutputKeyClass(Person.class);
job.setMapOutputValueClass(Text.class);
job.setReducerClass(DifferenceReduce.class);
job.setOutputKeyClass(Person.class);
job.setOutputValueClass(NullWritable.class);
FileInputFormat.addInputPaths(job, args[0]);
FileOutputFormat.setOutputPath(job, new Path(args[1]));
System.exit(job.waitForCompletion(true) ? 0 : 1);
}
}
运行代码1-20，运行结果如代码1-21
代码1-21
root@lejian:/data# hadoop jar difference.jar com.hadoop.mapreduce.Difference /data /output relationB ………… root@lejian:/data# hadoop fs -ls -R /output -rw-r--r-- 1 root supergroup 0 2017-01-08 08:59 /output/_SUCCESS -rw-r--r-- 1 root supergroup 69 2017-01-08 08:59 /output/part-r-00000 root@lejian:/data# hadoop fs -cat /output/part-r-00000 Tom male 21 Amy female 19 John male 19 Jojo female 26 Rose female 19
自然连接
如代码1-22，student集合的第一列是id，第二列是姓名，第三列是性别，第四列是年龄，grade集合第一列是id，第二列是科目，第三列是科目成绩，需要对student集合和grade集合做自然连接。在Map阶段将student和grade中每一条记录r作为value，而记录中的id作为key输出。在Reduce阶段则将同一键收集而来的数据根据它们的来源（student或grade）做笛卡尔积然后将结果输出
代码1-22中，将student集合和grade集合存储在HDFS下的data文件夹中
代码1-22
root@lejian:/data# cat student 1 Amy female 18 2 Tom male 19 3 Sam male 21 4 John male 19 5 Lily female 21 6 Rose female 20 root@lejian:/data# cat grade 1 Math 89 2 Math 75 4 English 85 3 English 95 5 Math 91 5 English 88 6 Math 78 6 English 99 2 English 80 root@lejian:/data# hadoop fs -put student /data root@lejian:/data# hadoop fs -put grade /data root@lejian:/data# hadoop fs -ls -R /data -rw-r--r-- 1 root supergroup 105 2017-01-08 09:59 /data/grade -rw-r--r-- 1 root supergroup 93 2017-01-08 09:59 /data/student
代码1-23
package com.hadoop.mapreduce;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.LongWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Mapper;
import org.apache.hadoop.mapreduce.lib.input.FileSplit;
public class NaturalJoinMap extends Mapper<LongWritable, Text, IntWritable, Text> {
private String fileName = "";
private Text val = new Text();
private IntWritable stuKey = new IntWritable();
protected void setup(Context context) throws java.io.IOException, InterruptedException {
FileSplit fileSplit = (FileSplit) context.getInputSplit();
fileName = fileSplit.getPath().getName();
};
protected void map(LongWritable key, Text value, Context context) throws java.io.IOException, InterruptedException {
String[] arr = value.toString().split(" ");
stuKey.set(Integer.parseInt(arr[0]));
val.set(fileName + " " + value.toString());
context.write(stuKey, val);
};
}
代码1-24
package com.hadoop.mapreduce;
import java.util.ArrayList;
import java.util.List;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Reducer;
public class NaturalJoinReduce extends Reducer<IntWritable, Text, Text, NullWritable> {
private Text student = new Text();
private Text value = new Text();
protected void reduce(IntWritable key, Iterable<Text> values, Context context) throws java.io.IOException, InterruptedException {
List<String> grades = new ArrayList<String>();
for (Text val : values) {
if (val.toString().contains("student")) {
student.set(studentStr(val.toString()));
} else {
grades.add(gradeStr(val.toString()));
}
}
for (String grade : grades) {
value.set(student.toString() + grade);
context.write(value, NullWritable.get());
}
};
private String studentStr(String line) {
String[] arr = line.split(" ");
StringBuilder str = new StringBuilder();
for (int i = 1; i < arr.length; i++) {
str.append(arr[i] + " ");
}
return str.toString();
}
private String gradeStr(String line) {
String[] arr = line.split(" ");
StringBuilder str = new StringBuilder();
for (int i = 2; i < arr.length; i++) {
str.append(arr[i] + " ");
}
return str.toString();
}
}
代码1-25
package com.hadoop.mapreduce;
import java.io.IOException;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.io.IntWritable;
import org.apache.hadoop.io.NullWritable;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapreduce.Job;
import org.apache.hadoop.mapreduce.lib.input.FileInputFormat;
import org.apache.hadoop.mapreduce.lib.output.FileOutputFormat;
public class NaturalJoin {
public static void main(String[] args) throws IOException, ClassNotFoundException, InterruptedException {
if (args == null || args.length != 2) {
throw new RuntimeException("请输入输入路径、输出路径");
}
Configuration conf = new Configuration();
Job job = Job.getInstance(conf);
job.setJobName("NaturalJoin");
job.setJarByClass(NaturalJoin.class);
job.setMapperClass(NaturalJoinMap.class);
job.setMapOutputKeyClass(IntWritable.class);
job.setMapOutputValueClass(Text.class);
job.setReducerClass(NaturalJoinReduce.class);
job.setOutputKeyClass(IntWritable.class);
job.setOutputValueClass(NullWritable.class);
FileInputFormat.addInputPaths(job, args[0]);
FileOutputFormat.setOutputPath(job, new Path(args[1]));
System.exit(job.waitForCompletion(true) ? 0 : 1);
}
}
运行代码1-25，运行结果如代码1-26
代码1-26
root@lejian:/data# hadoop jar naturalJoin.jar com.hadoop.mapreduce.NaturalJoin /data /output ………… root@lejian:/data# hadoop fs -ls -R /output -rw-r--r-- 1 root supergroup 0 2017-01-08 11:19 /output/_SUCCESS -rw-r--r-- 1 root supergroup 237 2017-01-08 11:19 /output/part-r-00000 root@lejian:/data# hadoop fs -cat /output/part-r-00000 1 Amy female 18 Math 89 2 Tom male 19 English 80 2 Tom male 19 Math 75 3 Sam male 21 English 95 4 John male 19 English 85 5 Lily female 21 English 88 5 Lily female 21 Math 91 6 Rose female 20 English 99 6 Rose female 20 Math 78