i am using Joomfish component it give error "500 not valid database" For my Component. after i transformed my data-base mysql to mysqli then it's working fine. Why??

Unless of course you're running PHP 5.3, the mysqli extension isn't installed automatically. You most likely haven't installed it, or it is not configures correctly.

Book the phpinfo() function to ascertain if it's installed and read about setting up mysqli to determine how you can install and/or configure it.