You Don't Know that Your DBMS Is Good
That could be true if you're a student or new on the job. That could be true especially if you're writing queries or programs that should work on more than one DBMS. You're most likely to encounter one of the following RDBMSs with the largest market shares (based on new license sales figures for the year 2001)  :
You might also run into DBMSs that are popular for Web work (like MySQL), for work with Borland products (like InterBase), for desktops (like Access), for mobile and Java interfacing (like Cloudscape), or for embedded systems (like Solid), or a host of small fry, like mSQL and gadfly. This book tries to be useful for the common denominator in all products. To use automotive analogies , it's not a " mechanic 's guide to tuning the 1999 Cadillac" book, it's a "driver's guide to optimizing performance of modern cars " bookeven if you have a manual transmission.