Describe a difficult problem you have had to face. With databases, the world is not perfect. Be prepared for corruption. Always backup your work. If you face a corrupted database learn how to fix it before a disaster occurs.
Databases are memory hogs.
Make sure your hardware can handle the software. The manufacturer will always give you the minimum requirements for good results. This is rarely enough. If you are working on a network, learn how your databases will perform during slowdowns. Be prepared to answer these questions for your users, because you will always hear from your users when things are not running well.