I found this solution : here
So…
class MainClassAddon{
public function __construct(){
register_activation_hook( __FILE__, array( $this, 'install' ) );
}
public function install(){
$row = $wpdb->get_results( "SELECT COLUMN_NAME FROM INFORMATION_SCHEMA.COLUMNS WHERE table_name="wp_t" AND column_name="c"" );
if(empty($row)){
$wpdb->query("ALTER TABLE wp_t ADD c INT(1) NOT NULL DEFAULT 1");
}
}
}
new MainClassAddon();
register_uninstall_hook( __FILE__, array( 'PluginNamespace\MainClassAddon', 'uninstall' ) );