主键的自动增长auto_increment

CREATE TABLE student(
    id int PRIMARY KEY AUTO_INCREMENT,
  name varchar(20) UNIQUE
);
INSERT INTO student (name) VALUES('mayun');
INSERT INTO student (name) VALUES('mahuateng');
INSERT INTO student (name) VALUES('liyanhong');
INSERT INTO student (name) VALUES('liuqiangdong');

INSERT INTO student (id,name) VALUES(3,'liyanhong');

当你删除了最后的第n条记录，那么再此后增加的记录序号会是n+1。