declare
    column_exists exception;
    pragma exception_init (column_exists , -01430);
begin
    execute immediate 'ALTER TABLE PTOVTA_COMISSAO
	ADD (
		INDBILHETESTROCADOS NUMBER (1,0) DEFAULT 1,
		INDBILHETESTRANSFERIDOS NUMBER (1,0) DEFAULT 1,
		INDBILHETESDEVCANORIGEM NUMBER (1,0)
	)';
    exception when column_exists then null;
end;