Create table Persons(
person_id number(8) constraint pk_Persons primary key,
person_pword varchar2(16) not null;

declare
user_id number(8);
user_pword varchar(16);
alert_button number;
begin
	
	select person_id, person_pword
	into user_id, user_pword
	from Persons
	where :signin.txt_username = Persons.person_id;
	
	if(user_pword =:login.txt_pword AND user_id =:signin.txt_username)
		then
		Go_block('MENU');--SPECIFY A BLOCK
		
	else
		alert_button := show_alert('Incorrect details');
	end if;
	end; 

Create table Grades(
grade_code number constraint pk_Grades primary key, 
grade varchar(4) check (result in ('fail','pass')),
person_id number,
unit_code number,
CONSTRAINT fk_Persons FOREIGN KEY (person_id) REFERENCES Persons (person_id),
CONSTRAINT fk_Units FOREIGN KEY (unit_code) REFERENCES Units (unit_code));

declare
	user_id number(8);
	usergrade_code number;
	usergrade varchar(4);
	user_unitcode number(6);
	alert_button number;
	
begin
	select person_id, grade_code, grade, unit_code
	into user_id, usergrade_code, usergrade, user_unitcode
	from Grades
	where Grades.unit_code.123456 = 'pass' AND  = :signin.txt_username = Persons.person_id; /* 123456 being the unit code in question */
	
	if Grades.unit_code.123456 = pass
		THEN
		Go_block('NEW_UNITS')
else 
		alert_button := show_alert('dont qualify for this unit');
	end if;
	end;

declare 
 ...
begin
  select ... 
    into ...
    from ...
    where ...;

  go_block(...);
exception
  when no_data_found then
    message('Your message');
    raise form_trigger_failure;
end;

select person_id, grade_code, grade, unit_code
	into user_id, usergrade_code, usergrade, user_unitcode
	from Grades
	where Grades.unit_code.123456 = 'pass' AND  = :signin.txt_username = Persons.person_id;

declare
:global.user_id '8';
user_pword varchar(16);
alert_button number;

begin
	
	select person_id, person_pword
	into :global.user_id, user_pword
	from Persons
	where :signin.txt_username = Persons.person_id;
	
	if(user_pword =:login.txt_pword AND user_id =:signin.txt_username)
		then
                :global.user_id:=user_id;		
                Go_block('MENU');--SPECIFY A BLOCK
		
	else
		alert_button := show_alert('Incorrect details');
	end if;
exception
  when no_data_found then
    message('Meaningful alert message');
    raise form_trigger_failure;

	end;