And when you say “Hadoop”, it really is a larger project. I assume you just meant the MapReduce part of it. The main parts:
1. HDFS (GFS Equivalent).
2. A MapReduce implementation.
3. And now HBase, a Bigtable database equivalent.

While many examples of the MapReduce floating around scan large flag files, the “there is no database” statement is not technically true. In reality you can create your own InputFormats and RecordReaders to read from X number of databases or whatever.

Specifically, the HBase has a table mapper that you can use with your MR. And if you really wanted, you could write up mysql (or whatever) input formats, input splits and record readers for your MR.