//MAPPER pseudo code
public static class DistinctMapper extends  Mapper<Object, Text, Text, NullWritable> {
        private Text col1 = new Text();
        private Text col2 = new Text();

        public void map(Object key, Text value, Context context) throws IOException, InterruptedException {

            // Logic to extract columns
            String C1  = extractColumn(value);
            String C2  = extractColumn(value);

            if (C2 != 'WhereCluaseValue') {  // filter value
                return;
            }
            // Mapper output key to the distinct column value
            col1.set(C1);
            // Mapper value as NULL
            context.write(col1, NullWritable.get());
        }
    }

//REDUCER pseudo code
public static class DistinctReducer extends Reducer<Text, NullWritable, Text, NullWritable> {
        public void reduce(Text key, Iterable<NullWritable> values, Context context) throws IOException, InterruptedException {
            // distinct column with a null value
            //Here we are not concerned about the list of values
            context.write(key, NullWritable.get());
        }
}