select unit_code, unit_name, mandatory
from unit_codes
where ...

... 
where unit_code not in (select unit_code
                        from prohibited_unit_codes 
                        where ...
                       )

Create table Units(
unit_code varchar2 (11) constraint pk_Units primary key,
unit_name varchar2(50) not null;

Create table listofinserts(
insertion_id number constraint pk_listofinserts primary key,
person_id number(8) constraint fk_Persons references Persons (person_id),
unit_code varchar2(11) constraint fk_Units references Units (unit_code));

Create table Grades(
grade_code number constraint pk_Grades primary key, 
grade varchar(4) check (result in ('fail','pass')),
person_id number,
unit_code varchar2,
CONSTRAINT fk_Persons FOREIGN KEY (person_id) REFERENCES Persons (person_id),
CONSTRAINT fk_Units FOREIGN KEY (unit_code) REFERENCES Units (unit_code));

Create table Prerequisites(
preunit_code varchar2(11),
unit_code varchar2(11),
PRIMARY KEY (preunit_code, unit_code),
CONSTRAINT fk_Units3 FOREIGN KEY (unit_code) REFERENCES Units(unit_code),
CONSTRAINT fk_Units4 FOREIGN KEY (preunit_code) REFERENCES Units(unit_code));

INSERT INTO Prerequisites (preunit_code, unit_code)
VALUES
('MATHS010101', 'MATHS020202');

INSERT INTO Prerequisites (preunit_code, unit_code)
VALUES
('MUSIC010101', 'MUSIC020202');

INSERT INTO Grades (grade_code, grade, person_id, unit_code)
VALUES
(Grades_seq.NEXTVAL, 'pass','12345678','MUSIC010101');

INSERT INTO Grades (result_id, result, person_id, module_id)
VALUES
(Grades_seq.NEXTVAL, 'fail','12345678','MATHS010101');

select q.unit_code,
       u.unit_name
from prerequisites q,
     units u
where q.preunit_code in (select g.unit_code
                         from grades g,
                              persons p
                         where g.grade = 'pass'
                           and g.person_id = p.person_id
                           and p.person_id = :form_block.person_id
                        )
  and u.unit_code = q.unit_code
order by u.unit_code

Create table Courses_Units(
course_code number(3),
unit_code varchar2(11),
PRIMARY KEY (course_code, unit_code),
CONSTRAINT fk_Courses FOREIGN KEY (course_code) REFERENCES Courses (course_code),
CONSTRAINT fk_Units2 FOREIGN KEY (unit_code) REFERENCES Units(unit_code));

INSERT INTO Courses_Units (course_code, unit_code)
VALUES
(101, 'MUSIC020202');

INSERT INTO Courses_Units (course_code, unit_code)
VALUES
(101, 'MATHS020202');

select q.unit_code,
       u.unit_name
from prerequisites q,
     units u
where q.preunit_code in (select g.unit_code
                         from grades g,
                              persons p
                         where g.grade = 'pass'
                           and g.person_id = p.person_id
                           and p.person_id = global.person_id)--Not selecting the logged in ID
  and u.unit_code = q.unit_code
order by u.unit_code

... and p.person_id = :global.person_id)
                      ^
                      |
                      HERE!

	select person_id, person_pword
	into :global.user_id, user_pword
	from Persons
	:signin.txt_username = Persons.person_id;
	and :signin.txt_pword = Persons.person_pword;

declare
	
	user_grade varchar(4) := 'pass';
	user_grade_code varchar2(11) := 'MATHS010101';
	alert_button number;
begin

select DISTINCT grade, unit_code
	into user_grade, user_grade_code
	from Grades
	where user_grade = grade 
	and user_grade_code = unit_code and :global.user_id = person_id;
	message('Please select your units for this course');
	Go_block('block_with_LOV');
exception 
when no data found then (meaningful message) 

where user_grade = grade

SELECT Courses_Units.unit_code 
FROM Courses_Units, Units 
WHERE Units.unit_code NOT IN ('COMPU020202')
AND Courses_Units.unit_code = Units.unit_code
AND course_code = '7777'
UNION
SELECT Courses_Units.unt_code
FROM Courses_Units, Units, Prerequisites
WHERE Units.unit_code = 'COMPU020202'
AND Prerequisites.preunit_code in (select unit_code
                         from Grades, Persons 
                         where grade = 'pass'
                         and Grades.person_id = Persons.person_id
   			and Persons.person_id = :global.user_id
		   )
AND Prerequisites.unit_code = Units.unit_code
AND Courses_Units.unit_code = Units.units_code
AND course_code = '7777'
GROUP BY Courses_Units.unit_code
HAVING count(*) = 2