在更新 mysql 之前触发？

Question

create table Property4Rent
(
    PropertyNo char(4),
    Street varchar(30),
    City varchar(30),
    Postcode varchar(10),
    Type_p varchar(10),
    Rooms int,
    Rent int,
    OwnerNo char(4),
    StaffNo varchar(4),
    BranchNo char(4)
);

INSERT INTO Property4Rent
VALUES('PA14', '10 Holland', 'Abend',NULL,'House', 6, 650, 'CO46','SA9','B007');
INSERT INTO Property4Rent
VALUES('PG16', '5 Novar', 'Glasgow',NULL,'Flat', 4, 450, 'CO93','SG14',NULL);
INSERT INTO Property4Rent
VALUES('PG21', '19 Dale', 'Glasgow',NULL,'House', 5, 600, 'CO87','SG37','B003');
INSERT INTO Property4Rent
VALUES('PG36', '2 Manor', 'Glasgow',NULL,'Flat', 3, 375, 'CO93','SG37','B003');
INSERT INTO Property4Rent
VALUES('PG4', '5 Lawrence', 'Glasgow',NULL,'Flat', 3, 350, 'CO40',NULL,'B003');
INSERT INTO Property4Rent
VALUES('PG97', '7 Muir', 'Abend',NULL,NULL, NULL, NULL, NULL,'SA9',NULL);
INSERT INTO Property4Rent
VALUES('PL94', '8 Argyll', 'London',NULL,'Flat', 4, 800, 'CO87','Sl41','B005');

创建一个触发器来实现以下约束：伦敦公寓有房间号 3人以上，最低房租必须800。格拉斯哥的公寓房间数量多 2人以上，最低租金300

Answer 1

要在 MySQL 中创建一个触发器来强制执行您指定的约束，您可以使用以下代码：

DELIMITER $$

CREATE TRIGGER property_rent_check
BEFORE INSERT ON Property4Rent
FOR EACH ROW
BEGIN
  IF NEW.City = 'London' AND NEW.Rooms < 4 AND NEW.Rent < 800 THEN
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'London apartments must have at least 4 rooms and rent of at least 800';
  END IF;

  IF NEW.City = 'Glasgow' AND NEW.Rooms < 3 AND NEW.Rent < 300 THEN
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Glasgow apartments must have at least 3 rooms and rent of at least 300';
  END IF;
END$$

DELIMITER ;

此代码创建一个名为 property_rent_check 的触发器，该触发器在将新行插入 Property4Rent 表之前执行。触发器检查新行的 City 值是否为“London”或“Glasgow”，如果是，它会检查 Rooms 和 Rent 值是否满足指定的约束。如果不满足约束，触发器将使用 SIGNAL 语句引发错误。

您可以通过尝试将违反约束的新行插入 Property4Rent 表来测试此触发器。例如，以下 INSERT 语句应该失败并显示一条错误消息：

INSERT INTO Property4Rent
VALUES('PG50', '1 Main St', 'London', NULL, 'Flat', 3, 700, 'CO93', 'SG37', 'B003');

错误信息应该是：“London apartments must have at least 4 rooms and rent of at least 800”。