mySQL8.0触发语句

Question

I am trying to create as following given:

Create a trigger on the Company table after an update that sets a company's budget to 0 if it is less than 0.

But I struggle with errors such as being unable to update, set the table. I know it is because of I need to change from 'after update' to 'before update'. But I cannot dismiss the expected code. What could I do?

The mySQL code I have is:

 delimiter $$

 drop procedure if exists after_company_update $$ 

 create trigger after_company_update after update on Company 

 for each row begin 

 update Company set budget = 0 where budget < 0; 

 end $$ 

 delimiter ;

Error: ER_CANT_UPDATE_USED_TABLE_IN_SF_OR_TRG: Can't update table 'Company' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.

Answer 1

You can't update the whole table which is already used by the trigger but you can update the column which is been modified.

Assuming your table has Primary Key which is company_id

Like

update Company 
set budget = 0 
where company_id=new.company_id and budget < 0;

Answer 2

'In an UPDATE trigger, you can use OLD.col_name to refer to the columns of a row before it is updated and NEW.col_name to refer to the columns of the row after it is updated' - https://dev.mysql.com/doc/refman/8.0/en/trigger-syntax.html

A before trigger would work but I'm not sure that it would be appropriate to have a trigger at all - do you really want to do this every time an update occurs to company (or only on specific occasions) , for example would it be appropriate if you changed address(assuming address is in the company table)

delimiter $$
 drop trigger if exists after_company_update $$ 
 create trigger after_company_update BEFORE update on Company 
 for each row begin 
   set NEW.budget = 0 where OLD.budget < 0; 
 end $$ 
DELIMITER ;

Answer 3

Such a trigger should logically be done before the update rather than afterwards:

delimiter $$

drop trigger if exists before_company_update $$ 

create trigger before_company_update after update on Company 
for each row
begin 
    if old.budget < 0 then
        set new.budget := 0
    end if;
end $$ 

delimiter ;

You can express this as an after update trigger:

create trigger after_company_update after update on Company 
for each row
    update Company
        set budget = 0
        where Company.Id = new.Id;

The syntax works, but the trigger doesn't. It returns the error:

Can't update table 'Company' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.

You can see this here .