sqlite trigger checking equality of new tuple values with subqueries - sql

Question

I have a database representing a school with tables such as courses, students, and enrollment. They have the following schemas:
CREATE TABLE Students(
id INT CHECK(id > 100000),
lName CHAR(20),
fName CHAR(10),
gender CHAR(1) CHECK(gender IN('F','M')),
dob DATE,
address CHAR(100),
phone INT CHECK(phone > 1000000000),
grade CHAR(1) CHECK(grade IN('K','1','2','3','4','5')),
PRIMARY KEY(id));
CREATE TABLE Courses(
id INT CHECK(id > 1000),
name CHAR(40),
grade CHAR(1) CHECK(grade IN('K','1','2','3','4','5')),
PRIMARY KEY(id)
);
CREATE TABLE Enrollment(
studentID INT REFERENCES Students(id),
courseID INT REFERENCES Courses(id),
semester CHAR(6) CHECK(semester IN('Fall','Spring')),
year INT,
letterGrade CHAR(1) CHECK(letterGrade IN('A','B','C','D','E')),
PRIMARY KEY(studentID,courseID,semester,year));
I want to create a trigger for the enrollment table that will check that the grade level of the student with the given studentID and the grade level of the course with the courseID are equal before the tuple is inserted. I've been working on this for a while and can't seem to get it to work. Could anyone point me in the right direction?
This is what I've come up with:
CREATE TRIGGER appropriateLevel
...> BEFORE INSERT ON Enrollment
...> FOR EACH ROW
...> BEGIN
...> SELECT RAISE(ABORT,'The student's grade does not match the grade level of the course.')
...> WHERE EXISTS(SELECT * FROM Students, Courses WHERE ((Students.grade = Courses.grade) AND (Students.id = NEW.studentID) AND (Courses.id = NEW.courseID)));
...> END;
Thanks!

Answer 1

Your trigger aborts when the grades match.
Anyway, you don't need to use EXISTS because you want to compare only one specific value from each parent table:
CREATE TRIGGER appropriateLevel
BEFORE INSERT ON Enrollment
FOR EACH ROW
BEGIN
SELECT RAISE(ABORT, 'The student''s grade does not match the course''s grade level.')
WHERE (SELECT grade FROM Students WHERE id = NEW.studentID) <>
(SELECT grade FROM Courses WHERE id = NEW.courseID);
END;