If you prefer Java to C, I recommend taking a look at the source for H2, a pure Java SQL database engine. It is surprisingly easy to follow.

I've contributed to the H2 source code a few times, and in the process I got to learn much about how a database engine is implemented.

H2 home: http://www.h2database.com/

H2 Github repo: https://github.com/h2database/h2database