As I know, that library was created for Arduino.
Not all the librarys created for Arduino will work "as they are" in ESP32.
It could be a good idea to look for "MySQL_Connection library" and "esp8266".
Sure you can use that library with some little changes...
And if it work with esp8266, it will work with esp32
(Esp8266 is older than esp32, so ther is much more information about it...)