declare
    column_exists exception;
    pragma exception_init (column_exists , -01430);
begin
    execute immediate 'ALTER TABLE EMPRESA ADD (CNAE VARCHAR2(7))';
    execute immediate 'ALTER TABLE EMPRESA ADD (TARBPE VARCHAR2(20))';
    execute immediate 'ALTER TABLE EMPRESA ADD (CRTBPE NUMBER(1))';
    exception when column_exists then null;
end;
