In your .c file, you can use relative path to include the .h file. For example, if .c file is in /d/e/f/g folder and .h file is in /a/b/c folder, then your #include statement has to be
#include "../../../../a/b/c/mysql.h"
This is acceptable if the number of slashes is around 1 or 2 (2 is a bit dirty already). And it is not good if you move the folders around later.
Alternatively, you can only specify the name of the .h file, and use -I <dir> flag where you replace <dir> with the path to the folder that has .h file. If you move the folders around, you can just change a bit in the the compilation command (usually in a makefile).