Trigger for 2 Tables - sql

Question

this is a homework question, just to make it clear.
This is the relational schema:
PaperInvolvement (paperNr, academicId, paperRole)
Academic (academicId, acadName, employer)
So (academicID) is the primary key for Academic and (paperNr, academicId) is the primary key for PaperInvolvement table.
Here is the trigger that I am asked to do:
On PaperInvolvement after insert, update
On Academic after update
Prevent any 2 academics who work for the same company are involved in the same paper in the opposite roles.
Use stored procedure or cover it completely in trigger
There are only 2 roles available in this table, which is Reviewer and Author
This is what I have done so far:
CREATE TRIGGER TR_PaperInvolvement_1
ON PaperInvolvement
AFTER INSERT, UPDATE
AS
IF EXISTS
(
SELECT a.academicId, paperRole, paperNr
FROM
(SELECT academicId
FROM Academic
GROUP BY employer, academicId) AS a
JOIN
(SELECT academicId, paperRole, paperNr
FROM PaperInvolvement
GROUP BY paperNr, academicId, paperRole) AS p_inv
ON a.academicId = p_inv.academicId
WHERE paperRole = 'Author' AND paperRole = 'Reviewer'
)
BEGIN
RAISERROR('Cannot have 2 Academics from the same company to work on
different roles for this paper.',16,1)
ROLLBACK TRANSACTION
END
GO
My question is, based on the requirements (what I have listed on the bullet-lists), is this the correct way to answer the question?

Answer 1

Try this
CREATE TRIGGER TR_PaperInvolvement_Modify
ON PaperInvolvement
AFTER INSERT, UPDATE
AS
begin
if exists
(
select P.paperNr, A.employer
from PaperInvolvement as P
inner join Academic as A on A.academicID = P.academicID
where P.paperNr in (select i.paperNr from inserted as i)
group by P.paperNr, A.employer
having
count(case when P.paperRole = 'Author' then 1 end) > 0 and
count(case when P.paperRole = 'Reviewer' then 1 end) > 0
)
begin
raiserror('Cannot have 2 Academics from the same company to work on different roles for this paper.', 16, 1)
rollback transaction
end
end