Compare two values from different tables - MySql - mysql

Question

I need to check before insert into 'year' column of ( Academic Report) that it's not less than the year of 'enrollment date' column in (student)
my tables :
CREATE TABLE STUDENT
(
BCN INT (10) ,
Enrollment_Date timestamp not null default CURRENT_TIMESTAMP,
primary key (BCN),
);
CREATE TABLE ACADEMIC_REPORT
(
Stud_Num INT(10) ,
Year year ,
primary key (Stud_Num , Year ),
foreign key (Stud_Num ) references STUDENT( BCN )
ON DELETE restrict ON UPDATE CASCADE
);
I've try this :
/* ACADEMIC_REPORT._Year Constraint "BEFORE INSERT" */
delimiter //
CREATE TRIGGER ReportYearBIN BEFORE INSERT ON ACADEMIC_REPORT
FOR EACH ROW
BEGIN
declare num int(10);
SET num= (SELECT BCN
FROM STUDENT , ACADEMIC_REPORT
WHERE BCN = ACADEMIC_REPORT.Stud_Num );
IF (ACADEMIC_REPORT.Year < YEAR(num.Enrollment_Date))
THEN
SIGNAL SQLSTATE '10000'
SET MESSAGE_TEXT = 'Error....!';
END IF;
END;
// delimiter ;
but it causes this error during insert
Error Code: 1054. Unknown column 'ACADEMIC_REPORT.Year' in 'field list'
How can I do that constraint ?
Using MySql workbench 6.3
Thank you in advance.

Answer 1

Okay, I created a select statement for the conditions that you want to generate the error on. If the student exists for this report and the report year is less than the students enrollment date it will generate one or more rows for the select query. The proceeding FOUND_ROWS() will then be greater than 0 which will trigger your error message.
CREATE TRIGGER ReportYearBIN BEFORE INSERT ON ACADEMIC_REPORT
FOR EACH ROW
BEGIN
SELECT * FROM STUDENT WHERE BCN=new.Stud_Num AND new._year < Enrollment_Date
IF (SELECT FOUND_ROWS() > 0)
THEN
SIGNAL SQLSTATE '10000'
SET MESSAGE_TEXT = 'Error....!';
END IF;
END;

Answer 2

I've figured out the problem :)
This statement is completely wrong,
YEAR(num.Enrollment_Date)
because it's not correct to write attribute.attribute, you can only use Table.Attribute.
So, here is the solution
/* ACADEMIC_REPORT._Year Constraint "BEFORE INSERT" */
delimiter //
CREATE TRIGGER ReportYearBIN BEFORE INSERT ON ACADEMIC_REPORT
FOR EACH ROW
BEGIN
declare Enroll timestamp;
SET Enroll = (SELECT Enrollment_Date
FROM STUDENT
WHERE BCN = new.Stud_Num);
IF ( new._year < YEAR(Enroll))
THEN
SIGNAL SQLSTATE '10000'
SET MESSAGE_TEXT = 'Error..The Report year cannot be less than the enrollment year..!';
END IF;
END;
// delimiter ;