When you work with PHP, you will surely have worked with MySQL also. I really like the MySQL DBMS, it is easy to work with, any question you have you can find the answer in the manual. But ofcourse not everything is nice about it, when working with load intensive systems, You might need to improve the speed of your queries.
If you want to know how to do these things perhaps looking at this video can help you.
Or if you rather want to read some thing similar go to 10 tips for optimizing mysql queries or a critic of the previous article 10 Tips for Optimizing MySQL Queries (That don’t suck), I would advise you to read the later of them as it has some more usefull advise compared to the first one. But as always, I will let you be the judge of that.