MapReduce的两表join一般操作

案例：(部门员工两表的join查询)

原始数据

员工表(emp):

 empno ename  job      mgr  hiredate   sal  comm deptno loc
 7499  allen  salesman 7698 1981-02-20 1600 300  30
 7782  clark  manager  7639 1981-06-09 2450      10
 7654  martin salesman 7698 1981-03-22 1250 1400 30     boston
 7900  james  clerk    7698 1981-03-20 950       30
 7788  scott  analyst  7566 1981-09-01 3000 100  20 

部门表(dep):

deptno dname     loc
 30     sales     chicago
 20     research  dallas
 10     accouting newyork

实现的功能类似于:select e.empno,e.ename,d.deptno,dname from emp e join dept d on e.deptno=d.deptno;
用join on后面的字段作为key
一对多关系

解析：

最后输出的结果包含两张表共四个属性(员工id，员工姓名，部门id(外键)，部门名称)

我们可以将部门id作为map传值的key，将四个属性构造一个JavaBean作为map传值的value，其中自定义的JavaBean中除了包含四个属性外，还应有区分是员工表还是部门表的字段flag.

1.JavaBean

/* *实现join的两张表通用的一个bean,并且bean中加一个通用的标识flag,用于区分两张表 *实现writableCompare接口(由于数据要在网络上传输必须序列化,hadoop处理的时候需要分组和排序) */public class Bean implements WritableComparable<Bean> {// 两个表共查询的属性private String empno = " ";private String empname = " ";private String depno = " ";private String depname = " ";private int flag = 0; // 0:部门 1：员工public Bean() {}public Bean(String empno, String empname, String depno, String depname,int flag) {super();this.empno = empno;this.empname = empname;this.depno = depno;this.depname = depname;this.flag = flag;}public Bean(Bean bean) {this.empno = bean.getEmpno();this.empname = bean.getEmpname();this.depno = bean.getDepno();this.depname = bean.getDepname();this.flag = bean.getFlag();}@Overridepublic void write(DataOutput out) throws IOException {// TODO Auto-generated method stub// 写数据out.writeUTF(empno);out.writeUTF(empname);out.writeUTF(depno);out.writeUTF(depname);out.writeInt(flag);}@Overridepublic void readFields(DataInput in) throws IOException {// TODO Auto-generated method stub// 读数据this.empno = in.readUTF();this.empname = in.readUTF();this.depno = in.readUTF();this.depname = in.readUTF();this.flag = in.readInt();}@Overridepublic String toString() {return "empno=" + empno + ", empname=" + empname + ", depno=" + depno+ ", depname=" + depname;}@Overridepublic int compareTo(Bean arg0) {// TODO Auto-generated method stubreturn 0;}public String getEmpno() {return empno;}public void setEmpno(String empno) {this.empno = empno;}public String getEmpname() {return empname;}public void setEmpname(String empname) {this.empname = empname;}public String getDepno() {return depno;}public void setDepno(String depno) {this.depno = depno;}public String getDepname() {return depname;}public void setDepname(String depname) {this.depname = depname;}public int getFlag() {return flag;}public void setFlag(int flag) {this.flag = flag;}}

2.Map类( map的输出的key为join时员工表的deptno,输出的value为对象Bean,reduce时 会将两个相同的key组成一起)

/* * map的输出的key为join时员工表的deptno,输出的value为对象Bean * reduce时 会将两个相同的key组成一起 *//* * 将emp和dep定义相同的bean来处理 */public class StaffDepMap extends Mapper<LongWritable, Text, IntWritable, Bean> {@Overrideprotected void map(LongWritable key, Text value,Mapper<LongWritable, Text, IntWritable, Bean>.Context context)throws IOException, InterruptedException {String line = value.toString();String[] str = line.split("\t");// 判断表的类型if (str.length == 3) {// 部门数据Bean dep = new Bean();dep.setDepno(str[0]);dep.setDepname(str[1]);dep.setFlag(0);// 传递部门数据context.write(new IntWritable(Integer.parseInt(str[0])), dep);} else {// 员工数据Bean emp = new Bean();emp.setEmpno(str[0]);emp.setEmpname(str[1]);emp.setDepno(str[7]);emp.setFlag(1);// 传递员工数据context.write(new IntWritable(Integer.parseInt(str[7])), emp);}}}

3.Reduce类(输入的即是两个表的depno)

public class StaffDepRedu extendsReducer<IntWritable, Bean, NullWritable, Text> {@Overrideprotected void reduce(IntWritable key, Iterable<Bean> values,Reducer<IntWritable, Bean, NullWritable, Text>.Context context)throws IOException, InterruptedException {Bean dep = null;List<Bean> emps = new ArrayList<Bean>();for (Bean bean : values) {if (bean.getFlag() == 0) {// 部门数据dep = new Bean(bean); // 重新构造对象} else {// 员工数据emps.add(new Bean(bean));}}// 给员工数据list添加部门的dnamefor (Bean emp : emps) {emp.setDepname(dep.getDepname());context.write(NullWritable.get(), new Text(emp.toString()));}}}

4.job类

public class StaffDepMain {public static void main(String[] args) throws Exception {Configuration conf = new Configuration();Job job = new Job(conf);job.setJarByClass(StaffDepMain.class);job.setMapperClass(StaffDepMap.class);job.setMapOutputKeyClass(IntWritable.class);job.setMapOutputValueClass(Bean.class);job.setReducerClass(StaffDepRedu.class);job.setOutputKeyClass(NullWritable.class);job.setOutputValueClass(Text.class);FileInputFormat.addInputPath(job, new Path(args[0]));FileOutputFormat.setOutputPath(job, new Path(args[1]));job.waitForCompletion(true);}}