mysql如何创建修改触发器

CREATE TRIGGER trigger_name
trigger_time trigger_event
ON table_name FOR EACH ROW
BEGIN
    -- 触发器执行的SQL语句
END;

CREATE TABLE employees (
    id INT PRIMARY KEY,
    name VARCHAR(100),
    salary DECIMAL(10, 2)
);

DELIMITER $$

CREATE TRIGGER salary_change_trigger
AFTER UPDATE ON employees FOR EACH ROW
BEGIN
    INSERT INTO salary_history (employee_id, old_salary, new_salary)
    VALUES (OLD.id, OLD.salary, NEW.salary);
END$$

DELIMITER ;

DROP TRIGGER salary_change_trigger;

DELIMITER $$

CREATE TRIGGER salary_change_trigger
AFTER UPDATE ON employees FOR EACH ROW
BEGIN
    IF NEW.salary < OLD.salary * 0.8 THEN
        SIGNAL SQLSTATE '45000'
        SET MESSAGE_TEXT = 'New salary cannot be less than 80% of the old salary';
    ELSE
        INSERT INTO salary_history (employee_id, old_salary, new_salary)
        VALUES (OLD.id, OLD.salary, NEW.salary);
    END IF;
END$$

DELIMITER ;