These are the books I recommend you read if you want to learn more about database design or expand your knowledge of SQL. Keep in mind that some of these books will be challenging because they are more technical in nature. Also, some authors assume that you have a fairly significant background in computers, databases, and programming.
Connolly, Thomas, and Carolyn Begg. Database Systems: A Practical Approach to Design, Implementation, and Management (6th ed.). Essex, England: Addison-Wesley, 2014.
Coronel, Carlos and Steven Morris. Database Systems: Design, Implementation, and Management (11th ed.). Stamford, CT: Cengage Learning, 2015.
Date, C.J. An Introduction to Database Systems (8th ed.). Boston, MA: Addison-Wesley, ...