Trigger to increment a column value after update in a column of same table

Question

I have a table with two columns:

C1  |  C2
 1  |   3

Whenever C1 gets updated to 0, C2 should be incremented by 1.

I have developed a trigger:

create trigger increment
after update on T1
begin
  if new.C1=0 then
    update T1 set c2 = c2 +1;
  end if;
end;

But this raises an error whenever I try to update column C1:

ERROR 1442 (HY000): Can't update table 'T1' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.

I have only one update trigger on this table. How can I make this work?

Answer 1

You can also change values using new. You just have to use not after but before trigger.

delimiter $$
create trigger increment
before update on T1
begin
    if new.C1=0 then
        set new.c2 = new.c2 +1;
    end if;
end $$
delimiter ;