1.创建新的数据表t_reader2，表结构与t_reader一致，设置return_id为整型，且为AUTO_INCREMENT和UNIUQE
CREATE TABLE t_reader2
(
reader_id INT  UNIQUE AUTO_INCREMENT,
reader_name VARCHAR(50),
reader_sex CHAR(2),
reader_birthday DATE,
reader_borrowtotal INT
);

2.为已存在的表t_press的press_id设置唯一键与自增。
ALTER TABLE t_press MODIFY press_id INT UNIQUE AUTO_INCREMENT; 

插入数据验证
INSERT INTO t_press(press_name) VALUES("清华大学出版社");
INSERT INTO t_press(press_name) VALUES("北京水利水电出版社");
SELECT * FROM t_press;
ALTER TABLE t_press AUTO_INCREMENT=100;
INSERT INTO t_press(press_name) VALUES("吉林出版社");
INSERT INTO t_press(press_name) VALUES("高等教育出版社");
SELECT * FROM t_press;
INSERT INTO t_press(press_iD) VALUES(101);
INSERT INTO t_press(press_id) VALUES("101");
INSERT INTO t_press(press_id) VALUES("102");
