Many excellent general database texts are available that cover the broad fields of relational databases, ER modeling, and SQL. Among the best are:
An Introduction to Database Systems, C.J. Date (Addison Wesley).
Database System Concepts, A. Silberschatz, H.F. Korth, and S. Sudarshan (McGraw-Hill).
Fundamentals of Database Systems, R. Elmasri and S.B. Navathe (Addison Wesley).
Database Management Systems, R. Ramakrishnan and J. Gehrke (McGraw-Hill).
For a coverage of SQL, several good books exist, but many are out of print. Currently available books include:
A Guide to the SQL Standard: A User's Guide to the Standard Database Language SQL, C.J. Date and H. Darwen (Addison Wesley). This book isn't for the beginner but does an excellent job of covering the standard in detail.
SQL-99 Complete, Really, P. Gulutzan and T. Pelzer (CMP Books). At present, MySQL almost supports the SQL-92 standard, but this book is an excellent and long introduction to SQL with many worked examples.
For MySQL, the manual.html file distributed with the installation is an excellent resource. Other books include:
Managing and Using MySQL, R.J. Yarger, G. Reese, and T. King (O'Reilly).
MySQL, P. DuBois (New Riders Publishing).