MYSQL主键自增用法

<?php
header('content-type:text/html;charset=utf-8');
$mysqli=new mysqli('localhost','root','root','stu');
$i=1;
// while ($i <= 100) {
//   $sql="insert into user(uname,uage) values ('hdw{$i}',$i)";
//   $mysqli->query($sql);
// $i++;
// }
$sql='select * from user';
$result=$mysqli->query($sql);
while ($row=$result->fetch_assoc()) {
  echo "学员名:".$row['uname']."\t年龄：".$row['uage']."<br/>";
}
 ?>

create table user(
     id integer primary key,
     firstname varchar(200) not null,
     lastname varchar(200) not null,
     age integer
 );
 alter table user modify id integer auto_increment ;
 alter table user modify id integer default '1';
 alter table user add createtime timestamp default current_timestamp;

 create table dd(
     id int primary key not null auto_increment,
     name varchar(20),
     time timestamp default current_timestamp
 );