In today’s Hadoop world , MapReduce can be seen as a complement to an RDBMS. MapReduce is a good fit for processes that need to analyse the whole dataset, in a batch operation, specially for ad-hoc analysis. An RDBMS is good for point queries or updates, where the dataset has been indexed to deliver low latency retrieval and update times of a relatively small amount of data. MapReduce suits applications where the data is written once , and read many times, whereas a relational database is good datasets that are continually updated.
Thanks to Anindita Basak
madhavan 