Question

I need a statement that allows to change some fields in table1 when table2 was updated.

I have currently the following code:

delimiter //
CREATE TRIGGER check BEFORE UPDATE ON table2
FOR EACH ROW
BEGIN
    IF NEW.Count <> OLD.Count THEN
        ...
    END IF;
END//
delimiter ;

However, table1 is very large and there are a lot of update events, so I’m afraid that this code can slowdown my server. So I want to know if there is some solution that will not look over all records in it, but only those updated?

Answer 1

The trigger will not “look” over all records. Because you declared the trigger as FOR EACH ROW only a single row will be dealt with in the trigger.

As the trigger is execute for each row, NEW and OLD reference a single row as, which means that the condition NEW.Count <> OLD.Count simply compares two (simple) column values