一个building对应多个rooms，building删除----级联删除相关的rooms
第一步， 创建buildings表，如下创建语句：
USE testdb;
CREATE TABLE buildings (
building_no INT PRIMARY KEY AUTO_INCREMENT,
building_name VARCHAR(255) NOT NULL,
address VARCHAR(255) NOT NULL
)ENGINE=InnoDB DEFAULT CHARSET=utf8;
第二步， 创建rooms表，如下创建语句：
USE testdb;
CREATE TABLE rooms (
room_no INT PRIMARY KEY AUTO_INCREMENT,
room_name VARCHAR(255) NOT NULL,
building_no INT NOT NULL,
FOREIGN KEY (building_no)
REFERENCES buildings (building_no)
ON DELETE CASCADE
)ENGINE=InnoDB DEFAULT CHARSET=utf8;