Jim Scott explains that Hadoop and relational databases solve different problems:
That’s the basics. Peeling back the onion more reveals other distinct differences, further making the case more strongly for a Hadoop-RDBMS coexistence strategy. RDBMS has the backing of the biggest names in the software industry, and as such has fostered an install base of IT talent probably second to none. RDBMS integrate very well with other systems, and represent a very mature technology having venerable, 40-year old roots. RDBMS are baked into the very fabric of just about every mid-to large sized IT organization in the world. Believe it – RDBMS aren’t going away any time soon, nor should they.
Relational databases have a strong mathematical footing which provides unparalleled data integrity. Hadoop has a strong mathematical footing which provides near-linear scale out. The key is knowing the problem you need to solve and how to integrate the results.