I think we can not compare these two as both stands far way. Hadoop is not a database, It is a framework that supports data parallelism. As it is parallel processing system it can process petabytes of data with in less time which can not be achieved by your traditional processing mechnism. Hadoop has not designed for data storage rather for data processing. Hadoop it self can store any data without HDFS support. HDFS is a distibuted file system that accepts data from Hadoop and give it back to Hadoop based on its need.
Relational data base is purely a storage mechanism. It can not process any data rather stores it for longer period.
Structured data is data that is organized into entities that have a defined format, such as XML documents or database tables that conform to a particular predefined schema. This is the realm of the RDBMS. but, map reduce works with semi structured data (spread sheet), structured data (image files, plain text) because the input keys and values for MapReduce are not an intrinsic property of the data, but they are chosen by the person analyzing the data.
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