Mysql触发器不适用于更新查询

Question

我有一个每周出勤的 Mysql 表..

CREATE TABLE IF NOT EXISTS `students_attendance` (
  `student_id` SMALLINT(5) UNSIGNED ZEROFILL NOT NULL,
  `month` TINYINT UNSIGNED NOT NULL,
  `w1` ENUM ('Absent', 'Present', 'Leave') NULL,
  `w2` ENUM ('Absent', 'Present', 'Leave') NULL,
  `w3` ENUM ('Absent', 'Present', 'Leave') NULL,
  `w4` ENUM ('Absent', 'Present', 'Leave') NULL,
  `w5` ENUM ('Absent', 'Present', 'Leave') NULL,
  `timestamp` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  PRIMARY KEY (`student_id`, `month`)
) ENGINE=InnoDB;

我正在使用触发器为每个月自动插入新行

DELIMITER $$
  CREATE TRIGGER update_students_attendance
  BEFORE UPDATE ON `students_attendance` FOR EACH ROW
  begin
      DECLARE new_month TINYINT;
      SET new_month = (SELECT month FROM students_attendance WHERE student_id = NEW.student_id AND month = NEW.month);
      IF new_month is NULL OR new_month = ''
      THEN
          INSERT INTO students_attendance (student_id, month) VALUES (NEW.student_id, NEW.month);
      END IF;
  END;
  $$
DELIMITER ;

但是当我使用下面的更新语句时，它不会插入任何行......

UPDATE students_attendance set `w1` = 'Absent' where `student_id` = '1' and `month` = '1'

谁能帮帮我？？

Answer 1

请检查下面提到的触发器。

DELIMITER $$
  CREATE TRIGGER update_students_attendance
  BEFORE UPDATE ON `students_attendance` FOR EACH ROW
  begin
      DECLARE new_month TINYINT DEFAULT 0;
      SELECT month into new_month FROM students_attendance WHERE student_id = NEW.student_id AND month = NEW.month;
      IF new_month is NULL OR new_month < 1 THEN 
          INSERT INTO students_attendance (student_id, month) VALUES (NEW.student_id, NEW.month);
      END IF;
  END;
  $$
DELIMITER ;