declare
    column_exists exception;
    pragma exception_init (column_exists , -01430);
begin
    execute immediate 'ALTER TABLE FORMA_PAGO ADD (INDPERCENTUALDESCONTO NUMBER(1,0))';
    exception when column_exists then null;
end;
/
declare
    column_exists exception;
    pragma exception_init (column_exists , -01430);
begin
    execute immediate 'ALTER TABLE FORMA_PAGO ADD (VALORDESCONTO NUMBER(10,0))';
    exception when column_exists then null;
end;
/
declare
    column_exists exception;
    pragma exception_init (column_exists , -01430);
begin
    execute immediate 'ALTER TABLE FORMA_PAGO ADD (PERCENTUALDESCONTO NUMBER(10,0))';
    exception when column_exists then null;
end;