在列中保存DATEDIFF的MySql问题

Question

我正在编写酒店数据库，并且对DATEDIFF函数有疑问。我有一个包含来宾ID，到达日期，出发时间的表，并且我希望DATEDIFF（出发时间，到达时间）写入同一表的另一列。我怎样才能做到这一点？我试图制作两个表，但是一次工作，当我添加另一个来宾时，每次弹出主键重复错误。

当我创建空列'length'并尝试保存选择时，在此列中显示错误。码：插入预订（length_of_stay）中，从预订中选择DATEDIFF（出发，到达）；错误：

Error Code: 1452. Cannot add or update a child row: a foreign key constraint fails (`input_qt_plus`.`reservations`, CONSTRAINT `fk_reservations_guests` FOREIGN KEY (`guests_id_guests`) REFERENCES `guests` (`id_guests`) ON DELETE NO ACTION ON UPDATE NO ACTION)

表格：

这是客人的桌子：

 CREATE TABLE IF NOT EXISTS `input_qt_plus`.`guests` (
      `id_guests` INT NOT NULL AUTO_INCREMENT,
      `first_name` VARCHAR(45) NOT NULL,
      `last_name` VARCHAR(45) NOT NULL,
      `email` VARCHAR(45) NOT NULL,
      `phone` VARCHAR(45) NOT NULL,
      `id_num` VARCHAR(45) NULL,
      PRIMARY KEY (`id_guests`))
    ENGINE = InnoDB;

这是有保留的表：

CREATE TABLE IF NOT EXISTS `input_qt_plus`.`reservations` (
  `id_res` INT NOT NULL AUTO_INCREMENT,
  `guests_id_guests` INT NOT NULL,
  `arrival` DATE NOT NULL,
  `departure` DATE NOT NULL,
  PRIMARY KEY (`id_res`),
  INDEX `fk_reservations_guests_idx` (`guests_id_guests` ASC),
  CONSTRAINT `fk_reservations_guests`
    FOREIGN KEY (`guests_id_guests`)
    REFERENCES `input_qt_plus`.`guests` (`id_guests`)
    ON DELETE NO ACTION
    ON UPDATE NO ACTION)
ENGINE = InnoDB;

Answer 1

尝试此查询：希望它能正常工作

INSERT INTO your_table (field1, field2, arrival, departure, length)
    VALUES (value1, value2, arrivalDate, departureDate, DATEDIFF(day,arrivalDate, departureDate))

Answer 2

由于要更新同一张表，因此应两次使用它。首先将列添加到表中（假设您将其命名为stay_for），然后像这样更新它-更新预订R内部联接保留R2在R2.id_res = R.id_res设置R.stay_for = DATEDIFF（R2.departure，R2.arrival）;