样例数据：

1 北京
2 天津
3 河北
4 山西
5 内蒙古
6 辽宁
7 吉林
8 黑龙江

1 2010 1962
1 2011 2019
2 2010 1299
2 2011 1355
4 2010 3574
4 2011 3593
9 2010 2303
9 2011 2347

现在我们需要做的事情是将这两份数据 进行 内连接查询 ,得到下面的结果集：

1 北京 2011 2019
1 北京 2010 1962
2 天津 2011 1355
2 天津 2010 1299
4 山西 2011 3593
4 山西 2010 3574

设计模型
在设计阶段的设想： 在map端对文件进行分别处理 ，把所有记录修改成 key, value 键值对形式，把 id 作为 key， value 值根据数据的来源进行分类 ， 表1 的：“a#” + name ，表2 的：“b#” + score
在reduce阶段，shuffle功能把数据打乱了，我们根据之前的 value 值得前端 ab 来区分数据来源，分别放入两个 LinkedList ，然后做笛卡尔积，一条条存起来

代码实现：

public class MyInnerJoinDemo extends Configured implements Tool {
	private static class MyMapper extends Mapper<LongWritable, Text, LongWritable, Text>{
		private String[] strs = null;
		private LongWritable outkey = new LongWritable();
		private Text outval = new Text();
		
		@Override
		protected void map(LongWritable key, Text value, Mapper<LongWritable, Text, LongWritable, Text>.Context context)
				throws IOException, InterruptedException {
			strs = value.toString().split("\t");
			context.getCounter("Line Quality Statistics", "Line Quality Statistics").increment(1);
			if (null != strs && (strs.length == 2 || strs.length == 3)) {
				context.getCounter("Line Quality Statistics", "Nice Quality Statistics").increment(1);
				if (strs.length == 2) {
					outkey.set(Long.parseLong(strs[0].trim()));
					outval.set("a" + "\001" + strs[1]);
					context.write(outkey, outval);
				} else {
					outkey.set(Long.parseLong(strs[0].trim()));
					outval.set("b" + "\001" + strs[1] + "\t" + strs[2]);
					context.write(outkey, outval);
				}
				
			}
			else {
				context.getCounter("Line Quality Statistics", "Bad Quality Statistics").increment(1);
			}
			
		}
	}
	private static class MyReducer extends Reducer<LongWritable, Text, AreaWritable, NullWritable>{
		
		private AreaWritable outkey = new AreaWritable();
		// 存储单个Area字段值
		private Integer aid;
		private String aname;
		private Integer year;
		private Long count;
		private String year_count;
		// 存储values字符串
		private String value = "";
		// 存储year_count字符串
		private List<String> list = null;
		// 存储values字符串
		private List<String> list2 = null;
		// 存储分割过的value
		private String[] strs = null;
		
		
		@Override
		protected void reduce(LongWritable key, Iterable<Text> values,
				Reducer<LongWritable, Text, AreaWritable, NullWritable>.Context context) throws IOException, InterruptedException {
			
			/*
			 * 由于迭代器共用一个对象，因此value输出的时候都会变成最后调用的那个对象
			 * value = IteratorUtils.toList(values.iterator()).toString();
			 * System.out.println(key + "\t" + value);
			 * 下面每行输出都是同一个元素
			 * [b
2010	1962, b
2010	1962, b
2010	1962]
			 * [a
天津, a
天津, a
天津]
			 * ……
			 * 因此需要在遍历迭代器的时候把每个元素存储到一个新的list集合中
			 */
			
			// 初始化变量
			aid = (int)(key.get());
			aname = "";
			year = 0;
			count = 0L;
			list = new ArrayList<String>();
			list2 = new ArrayList<String>();
			
			for (Text text : values) {
				list2.add(text.toString());
			}
			value = list2.toString().replace("[", "").replace("]", "");
			// indexOf返回字符串在value中的位置，返回值>-1说明字符串中有指定字符
			if (value.indexOf("a") > -1 && value.indexOf("b") > -1) {
				strs = value.split(",");
				for (String s : strs) {
					if (s.trim().startsWith("a")) {
						aname = s.split("\001")[1];
					}
					if (s.trim().startsWith("b")) {
						year_count = s.split("\001")[1];
						list.add(year_count);
					}
				}
				
			}
			// 循环list 最终输出
			for (String s1 : list) {
				
				year = Integer.parseInt(s1.split("\t")[0].trim());
				count = Long.parseLong(s1.split("\t")[1].trim());
				outkey.setAid(aid);
				outkey.setAname(aname);
				outkey.setYear(year);
				outkey.setCount(count);
				// 输出
				context.write(outkey, NullWritable.get());
				
			}
			
			
		}
	}

	@Override
	public int run(String[] args) throws Exception {
		
		Configuration conf = this.getConf();
		Job job = Job.getInstance(conf, "MyJob_1");
		job.setJarByClass(MyInnerJoinDemo.class);
		job.setInputFormatClass(TextInputFormat.class);
		// 配置Map和Reduce
		job.setMapperClass(MyMapper.class);
		job.setReducerClass(MyReducer.class);
		// 设置Mapper输出
		job.setMapOutputKeyClass(LongWritable.class);
		job.setMapOutputValueClass(Text.class);
		// 设置最终输出
		job.setOutputKeyClass(AreaWritable.class);
		job.setOutputValueClass(NullWritable.class);
		
		job.setOutputFormatClass(TextOutputFormat.class);
		
		// 输入输出目录
		Path in = new Path(args[0]);
		Path out = new Path(args[1]);
		// 设置目录
		FileInputFormat.addInputPath(job, in);
		FileOutputFormat.setOutputPath(job, out);
		
		FileSystem fs = FileSystem.get(conf);
		// 输出目录必须不存在 若存在 删除！
		if (fs.exists(out)) {
			fs.delete(out,true);
			System.out.println(job.getJobName()+"的输出目录已删除！");
		}
		
		Long start = System.currentTimeMillis();
		// 等待job执行结束
		boolean con = job.waitForCompletion(true);
		Long end = System.currentTimeMillis();
		
		if (con) {
			System.out.println("success!");
			System.out.println("任务耗时：" + ((end - start)/1000) + "秒");
		}else {
			System.out.println("error!");
		}
		
		return 0;
		
	}
	public static void main(String[] args) {
		try {
			System.exit(ToolRunner.run(new MyInnerJoinDemo(), args));
		} catch (Exception e) {
			
			// TODO Auto-generated catch block
			e.printStackTrace();
			
		}
	}

}

Q.E.D.