This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I would classify Hadoop as a "extreme record processing tool". Hadoop doesn't have transactional semantics which are offered by relational databases. It's really a batch-based system that allows you to work with large data volumes at scale.