How to solve this exercise about Oracle Triggers - oracle

Question

I have to solve this exercise about triggers:
Consider the following relational database schema used to represent
project information:
Person (ID, Surname, Name, Nationality)
Project (Name, Manager,
StartingYear, NumPeopleInvolved, International)
Personnel (Project, PersonID)
Specify the triggers required in Oracle to maintain the following
integrity constraints:
a) The number of people involved in a project (attribute
NumPeopleInvolved) must be consistent with the number of tuples
entered in Personnel for that project
b) If the project is international (the International attribute
assumes only two values) then the project must involve at least two
people of different nationalities
I have a problem with the b) part.
I don't know how to handle the case in which a given Project has no people involved. If I try to insert the first people, I can not have two people of different nationalities since I have only one people.
How should this situation be handled?
Should I use a statement level trigger? I have not experience with triggers so I still haven't understood well what I can / I can't do with one kind of trigger.
I tried this way but it's clearly not working as it should:
CREATE TRIGGER InsertPersonnelInternational
AFTER INSERT ON Personnel
FOR EACH ROW
BEGIN
SELECT ProjectName
FROM Personnel INNER JOIN Project
WHERE PersonID = :new.ID Project = Name
SELECT International
FROM Personnel INNER JOIN Project
ON Project = Name
SELECT COUNT(*) AS NumPersonnel
FROM Personnel
WHERE Project = :new.Project
IF NumPersonnel >= 1 THEN
BEGIN
SELECT COUNT(*) AS NumNationalities
FROM Personnel INNER JOIN Person
ON Project = ProjectName
GROUP BY Nationality
IF International THEN
IF NumNationalities = 1 Then
BEGIN
raise_application_error(-1)
END
ELSE
IF NumNationalities <> 1 THEN
BEGIN
raise_application_error(-1)
END
END
END
END

Answer 1

When you have a row-level trigger on table Personnel then you cannot run any SELECT on table Personnel within the trigger - you will get an ORA-04091: table PERSONEL is mutating ... error.
I think your teacher is expecting something like this:
CREATE TRIGGER ProjectConsistency
BEFORE INSERT OR UPDATE ON PROJECT
FOR EACH ROW
p_count INTEGER;
n_count INTEGER;
BEGIN
SELECT COUNT(*)
INTO p_count
FROM Personnel
WHERE PROJECT = :new.NAME;
IF :new.NumPeopleInvolved <> p_count THEN
RAISE_APPLICATION_ERROR(-20010, 'The number of people involved in a project must be consistent with the number of tuples entered in Personnel for that project');
END IF;
IF :new.International = 'YES' THEN
SELECT COUNT(DISTINCT Nationality)
INTO n_count
FROM Personnel
WHERE PROJECT = :new.NAME;
IF n_count < 2 THEN
RAISE_APPLICATION_ERROR(-20010, 'The project must involve at least two people of different nationalities')
END IF;
END IF;
END;
In reality you would not implement such requirement with a trigger, you would use a PL/SQL procedure.
Attribute NumPeopleInvolved is useless, i.e. redundant. Typically you would solve it by
UPDATE PROJECT proj
SET NumPeopleInvolved =
(SELECT COUNT(*)
FROM Personnel p
WHERE PROJECT = :new.NAME)
WHERE NAME = :new.NAME;
Such an update could be done by a trigger, for example.
Actually you would need similar triggers also on table Personnel and Person, because the personel/persons may change and project would become inconsistent. I don't know whether this should be considered by the exercise.
Imagine, on person gets released, i.e. deleted from table Person:
would the application raise an error - person cannot be released (what happens if the person dies by Corona :-))?
would be project be invalid?
would be project be automatically updated?
Then, you should never raise errors like raise_application_error(-1) - always let the user know what went wrong!

Answer 2

The best way to do this is with a compound trigger. With a compound trigger we avoid the problem of mutating tables which we would get from a row level trigger on PERSONNEL.
We keep track of each project which is referenced by each affected row in a DML statement (insert, update, delete) in an array. At the end of the statement we query those projects to find whether the project is international, and if it is to check the nationalities of its assigned personnel.
It might look like this:
CREATE OR REPLACE TRIGGER international_project_trg
FOR insert or update or delete ON personnel
COMPOUND TRIGGER
-- Global declaration
type project_t is table of number index by personnel.project%type;
g_project project_t;
BEFORE EACH ROW IS
BEGIN
CASE
-- we don't care about the value here, we just what a set of distinct projects
WHEN INSERTING THEN
g_project(:new.project) := 1;
WHEN UPDATING THEN
g_project(:new.project) := 1;
WHEN DELETING THEN
g_project(:old.project) := 1;
END CASE;
END BEFORE EACH ROW;
AFTER STATEMENT IS
l_project personnel.project%type;
l_country_cnt pls_integer;
l_people_cnt pls_integer;
BEGIN
l_project := g_project.first();
while l_project is not null loop
select count(distinct ppl.nationality)
,count(*)
into l_country_cnt
,l_people_cnt
from personnel per
join project prj on per.project = prj.name
join person ppl on per.personid = ppl.id
where per.project = l_project
and prj.international = 'Y';
if l_people_cnt <= 1 then
-- either not international project or only one assigned person
-- so we don't care
null;
elsif l_country_cnt <= 1 then
raise_application_error(-20999, l_project ||' must have multi-national team membership');
end if;
l_project := g_project.next(l_project);
end loop;
END AFTER STATEMENT;
END international_project_trg;
Here is a working demo on db<>fiddle. You can see that although the trigger allows for an international project to have only one assigned person it throws an error when we add a second person of the same nationality. We can solve this by inserting rows in a special order, or better by inserting a set of rows. This is a problem with apply such business rules.
You can use the same approach (in the same trigger) to check that whether the number of assigned personnel meets the Project.NumPeopleInvolved rule.
Note: compound triggers arrived in Oracle 11gR1.

Answer 3

I think the following should work with insertions, deletions and updates on table Personnel. It simply check and update the international consistency for each project whether the table Personnel is altered.
CREATE TRIGGER UpdateInternationalProject
AFTER INSERT OR UPDATE OR DELETE ON Personnel
BEGIN
SELECT name, international
FROM Project
AS ProjectInternational;
FOR projectInfo IN ProjectInternational
LOOP
SELECT COUNT(DISTINCT nationality)
AS numNationalities
FROM Personnel INNER JOIN Person
ON personId = id
WHERE project = projectInfo.name;
IF numNationalities = 1 THEN
IF projectInfo.international THEN
UPDATE Project
SET international = 0
WHERE name = projectInfo.name;
END IF;
ELIF numNationalities > 1 THEN
IF NOT projectInfo.international THEN
UPDATE Project
SET international = 1
WHERE name = projectInfo.name;
END IF;
END IF;
END LOOP;
END;