mysql> drop database if exists myschool;
mysql> create database if not exists myschool;
mysql> use myschool;
Database changed
mysql> drop table if exists tb_testinfo;
#create table 表名 (字段名 类型 长度 <约束，默认值，注释>);
mysql> 
create table tb_testinfo
(
  test_id integer(10),
  test_name varchar(30)
 );

Query OK, 0 rows affected, 1 warning (0.40 sec)

drop table if exists tb_testinfo;
create table tb_testinfo
(
  test_id integer(10) auto_increment primary key comment '主键id',
  test_name varchar(30) not null comment '名称',
  test_pwd varchar(20) not null default '888888' comment '密码'
 );

 create table tb_user
(
  user_id int auto_increment primary key comment '用户编号',
  user_name varchar(30) not null ,
  user_birthday date,
  user_gender char(3),
  user_state tinyint(1) not null,
  user_height decimal(4,1) not null,
  user_desc text
 );

#新增字段
mysql>  alter table tb_user  add address varchar(100) not null default '' comment '地址';
#修改字段类型
mysql> alter table tb_user MODIFY address varchar(50) not null default '' comment '地址';
mysql> alter table tb_user change address addr varchar(100) not null default '' comment '地址';
#字段查看
mysql> desc tb_user;
+---------------+--------------+------+-----+---------+----------------+
| Field         | Type         | Null | Key | Default | Extra          |
+---------------+--------------+------+-----+---------+----------------+
| user_id       | int          | NO   | PRI | NULL    | auto_increment |
| user_name     | varchar(30)  | NO   |     | NULL    |                |
| user_birthday | date         | YES  |     | NULL    |                |
| user_gender   | char(3)      | YES  |     | NULL    |                |
| user_state    | tinyint(1)   | NO   |     | NULL    |                |
| user_height   | decimal(4,1) | NO   |     | NULL    |                |
| user_desc     | text         | YES  |     | NULL    |                |
| addr          | varchar(100) | NO   |     |         |                |
+---------------+--------------+------+-----+---------+----------------+
8 rows in set (0.00 sec)
#删除字段
mysql> alter table tb_user drop addr;
Query OK, 0 rows affected (0.32 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc tb_user;
+---------------+--------------+------+-----+---------+----------------+
| Field         | Type         | Null | Key | Default | Extra          |
+---------------+--------------+------+-----+---------+----------------+
| user_id       | int          | NO   | PRI | NULL    | auto_increment |
| user_name     | varchar(30)  | NO   |     | NULL    |                |
| user_birthday | date         | YES  |     | NULL    |                |
| user_gender   | char(3)      | YES  |     | NULL    |                |
| user_state    | tinyint(1)   | NO   |     | NULL    |                |
| user_height   | decimal(4,1) | NO   |     | NULL    |                |
| user_desc     | text         | YES  |     | NULL    |                |
+---------------+--------------+------+-----+---------+----------------+
#修改表名
mysql> alter table tb_user rename to t_user;
#修改表引擎（show create table t_user; 查看建表语句）
mysql> alter table t_user engine=myisam;