Use Case statement inside if else block in mysql triggers - mysql

Question

I am new to the mysql triggers.
Let assume if i have a table name teachers contains
teacher_id,
teacher_name,
description,
student_id,
class,
student_year
I want to create a AFTER INSERT and AFTER UPDATE triggers on teachers table.
for that i have a table name called teacherslogs with id,teacher_id,student_year,class,student_id columns
so whenever record insert into teachers table it should insert into teacherslogs.(for this trigger able to write)
But for after update trigger facing issue.
Requirement:
whenever student_id changed in teachers table we need to insert a record into teacherslogs with along another columns changes in the teachers table for that record
for this i have written like this
IF ((NEW.student_id <=> OLD.student_id) = 0) Then
select student_id,class,student_year into student_id_,class_,year_ from `employees`.`teachers` where teacher_id = OLD.teacher_id;
Insert into `employees`.`teacherlogs`(teacher_id,student_year,class,student_id) values (OLD.teacher_id,year_,class_,student_id_);
END IF;
But whenever changes in other columns like class,student_year we need to update the teacherslogs table for that record where teacher_id and student_id
Final Trigger Query
create trigger `updateteacherlogs`
AFTER UPDATE ON `employees`.`teacher` FOR EACH ROW
BEGIN
DECLARE teacher_id_ int(11);
Declare student_id_ int(11);
Declare class_ varchar(150);
Declare year_ int(11);
Declare dummy tinyint;
IF ((NEW.student_id <=> OLD.student_id) = 0) Then
select student_id,class,student_year into student_id_,class_,year_ from `employees`.`teachers` where teacher_id = OLD.teacher_id;
Insert into `employees`.`teacherlogs`(teacher_id,student_year,class,student_id) values (OLD.teacher_id,year_,class_,student_id_);
ELSE
CASE WHEN (NEW.student_year <=> OLD.student_year) = 0
THEN update `teacherlogs` set student_year = NEW.student_year where student_id=OLD.student_id and teacher_id = OLD.teacher_id;
WHEN (NEW.class <=> OLD.class) = 0
THEN update `teacherlogs` set class = NEW.class where student_id=OLD.student_id and teacher_id = OLD.teacher_id
ELSE select #dummy END
end if;
END
But i am getting an exception at end if.
is that correct way to solve my problem or any other ways can we do?

Answer 1

The reason for your syntax error is that your CASE statement needs to end in END CASE;, not just END.
I don't think that the CASE statement actually works for your requirements though as if NEW.student_year <=> OLD.student_year then NEW.class will not get compared against OLD.class to see if that value also needs to be updated. That is because (from the manual):
For the second syntax, each WHEN clause search_condition expression is
evaluated until one is true, at which point its corresponding THEN
clause statement_list executes
You can get around that by just replacing the ELSE CASE ... with an ELSEIF clause:
ELSEIF NEW.student_year <=> OLD.student_year OR NEW.class <=> OLD.class THEN
UPDATE teacherlogs
SET student_year = NEW.student_year, class = NEW.class
WHERE student_id = OLD.student_id AND teacher_id = OLD.teacher_id;
This should work because (based on my understanding from your question of how your database is configured) the value of student_year and class in teacherlogs should be the same as OLD.student_year and OLD.class and if they are the same as NEW.student_year and NEW.class the values won't get updated.
If my understanding is incorrect, you will need modify the update to have conditions on each value:
ELSE
UPDATE teacherlogs
SET student_year = IF(NEW.student_year <=> OLD.student_year, NEW.student_year, student_year),
class = IF(NEW.class <=> OLD.class, NEW.class, class)
-- repeat for all columns
WHERE student_id = OLD.student_id AND teacher_id = OLD.teacher_id;
END IF;