The WP_DEBUG option controls the reporting of some errors and warnings and enables use of the WP_DEBUG_DISPLAY and WP_DEBUG_LOG settings. The default boolean value is false.
define( 'WP_DEBUG', true );
Database errors are printed only if WP_DEBUG is set to true. Database errors are handled by the wpdb class and are not affected by PHP’s error settings.
Setting WP_DEBUG to true also raises the error reporting level to E_ALL and activates warnings when deprecated functions or files are used; otherwise, WordPress sets the error reporting level to E_ALL ^ E_NOTICE ^ E_USER_NOTICE.