Ask HN: Understanding How Databases Work

SQL based databases are complex, and we have a vast amount of different flavors available.

If someone fresh from university ask you for resources to understand basic concepts such as query parsing, query optimization, execution plans, etc

What kind of resources will you share with them? I expect to see people here who have been developing complex database systems, how do you get started? How is this different to other software engineering projects? Did you enjoy(ed) the process? What did you do to get better at it?