How to control column totals in mySQL using trigger?

Question

I want to use Trigger in Mysql to be able to control "evaluation_weight" column in "evaluation_criteria" table. If the sum of the "evaluation_weight" column >= 100 after update/insert, it will not be possible to update/insert the newly entered value.

Here is the picture of "evaluation_criteria" table

Answer 1

You would need two triggers with one for update and another one for insert.

delimiter //
drop trigger if exists check_evaluation_insert ;
create trigger check_evaluation_insert before insert on evaluation_criteria for each row 
begin
if  (select ifnull(sum(evaluation_weight),0) from evaluation_criteria) + new.evaluation_weight>= 100 then
signal SQLSTATE VALUE '99999' SET MESSAGE_TEXT = 'The limit of evaluation_weight has been reached. INSERT fails. ';
end if;
end//

drop trigger if exists check_evaluation_update ;
create trigger check_evaluation_update before update on evaluation_criteria for each row 
begin
if  (select ifnull(sum(evaluation_weight),0) from evaluation_criteria) + new.evaluation_weight - old.evaluation_weight>= 100 then
signal SQLSTATE VALUE '99999' SET MESSAGE_TEXT = 'The limit of evaluation_weight has been reached. UPDATE fails. ';
end if;
end//

delimiter ;