Trigger to update a table after an insert in a different one - mysql

Question

I am executing this query:
INSERT INTO Takes(Student_SSN_N,Exam_Couse_ID,Exam_ID)
SELECT "xxx", "y", "z"
FROM Student as S, Exam as E
WHERE EXISTS (SELECT *
FROM Follows
WHERE S.Student_SSN = "xxx"
AND S.Student_SSN = Follows.Student_SSN_N
AND Follows.Course_ID = y
)
AND EXISTS (SELECT *
FROM Follows
WHERE Follows.Course_ID = y
AND E.Course_ID = y
AND E.Exam_ID = z
)
(Values x, y, z are given by the user in a java application)
It works, the student that has to take the exam is correctly added to the table after checking if he actually follows the course and if the exam relates to the course itself. I'm just performing a check on the Exam table.
In the Exam table I maintain a "student number", which tells you how many students need to take the exam. I want to update this number every time I insert a new student in the Takes table. So I thought about using a trigger, and I wrote this:
DELIMITER $$
CREATE TRIGGER `Takes_after_insert` AFTER INSERT ON `Takes`
FOR EACH ROW
UPDATE Exam
SET Exam.Student_Num = Exam.Student_Num + 1
WHERE Takes.Course_ID = Exam.Course_ID and
Take.Exam_ID = Exam.Exam.ID;
It's a simple trigger (I haven't studied anything more complex yet). But I get this error:
Error Code: 1442. Can't update table 'Exam' in stored
function/trigger because it is already used by statement which invoked
this stored function/trigger.
(I also tried pointing the trigger to a different table just to see if the logic of it was wrong, and it worked)
Ok, I can't update the Exam table because I'm using it in the query that launches the trigger. But the how can I update the table?

Answer 1

You don't need the exam table in the insert. In fact, the logic seems quite hard to follow, but I think you are just checking two conditions and inserting one row. You can check the conditions as subqueries in the FROM and use CROSS JOIN . . . if either returns no rows, then the CROSS JOIN returns no rows.
So, I think this insert does what you want:
INSERT INTO Takes(Student_SSN_N, Exam_Couse_ID, Exam_ID)
SELECT "xxx", "y", "z"
FROM (SELECT 1
FROM Follows f
WHERE f.Student_SSN_N = "xxx" AND
f.Course_ID = y AND
) s CROSS JOIN
(SELECT 1
FROM Follows f
WHERE f.Course_ID = y AND
E.Exam_ID = z
) e;
The structure of the Follows table is quite curious, containing exams, students, and courses. But that is not the question you are asking.
As mentioned in the comment, you want new in the trigger rather than exams.