Left anti join

Left anti join gives only those rows from the left hand side table based that are not present in the right hand side table. Use this when you want to keep rows from the left table only when not present in right table. This provides very good performance, as only one table is fully considered and the other is only checked for the join condition:

We will consider the cities and temperatures if the cityID has only, name and no temperature records, as shown in the following code:

private static class LeftAntiJoinReducer
        extends Reducer<Text, Text, Text, IntWritable> {
    private IntWritable result = new IntWritable();
    private Text cityName = new Text("Unknown");
    public void reduce(Text key, Iterable<Text> values,
                       Context context) throws IOException, InterruptedException {
        int sum = 0;
        int n = 0;

        for (Text val : values) {
            String strVal = val.toString();
            if (strVal.length() <=3)
            {
                sum += Integer.parseInt(strVal);
                n +=1;
            } else {
                cityName = new Text(strVal);
            }
        }
        if (n==0 ) {
            if (n==0) n=1;
            result.set(sum / n);
            context.write(cityName, result);
        }
    }
}

The output will be as shown in the following code:

Las Vegas 0 // city of Las vegas has no temperature measurements in temperature.csv