修改表结构创建索引

mysql> create database feiyu_bbs;
Query OK, 1 row affected (0.00 sec)

mysql> use feiyu_bbs;
Database changed
mysql> create table bbs_user(
    -> id int unsigned not null auto_increment primary key,
    -> username varchar(32) not null unique,
    -> password char(32) not null,
    -> rtime int not null default 0,
    -> rip int not null default 0
    -> )engine=myisam default charset=utf8;
Query OK, 0 rows affected (0.01 sec)

mysql> desc bbs_user;
+----------+------------------+------+-----+---------+----------------+
| Field    | Type             | Null | Key | Default | Extra          |
+----------+------------------+------+-----+---------+----------------+
| id       | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(32)      | NO   | UNI | NULL    |                |
| password | char(32)         | NO   |     | NULL    |                |
| rtime    | int(11)          | NO   |     | 0       |                |
| rip      | int(11)          | NO   |     | 0       |                |
+----------+------------------+------+-----+---------+----------------+
5 rows in set (0.03 sec)

mysql> create table bbs_user1(
    -> id int unsigned not null auto_increment,
    -> username varchar(32) not null,
    -> password char(32) not null,
    -> primary key(id),
    -> unique bbs_user_username(username)
    -> )engine=myisam default charset=utf8;
Query OK, 0 rows affected (0.01 sec)

mysql> desc bbs_user1;
+----------+------------------+------+-----+---------+----------------+
| Field    | Type             | Null | Key | Default | Extra          |
+----------+------------------+------+-----+---------+----------------+
| id       | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(32)      | NO   | UNI | NULL    |                |
| password | char(32)         | NO   |     | NULL    |                |
+----------+------------------+------+-----+---------+----------------+
3 rows in set (0.03 sec)

mysql> create table bbs_user2(
    -> id int unsigned not null,
    -> username varchar (32)not null
    -> )engine=myisam default charset=utf8;
Query OK, 0 rows affected (0.01 sec)

mysql> desc bbs_user2;
+----------+------------------+------+-----+---------+-------+
| Field    | Type             | Null | Key | Default | Extra |
+----------+------------------+------+-----+---------+-------+
| id       | int(10) unsigned | NO   |     | NULL    |       |
| username | varchar(32)      | NO   |     | NULL    |       |
+----------+------------------+------+-----+---------+-------+
2 rows in set (0.03 sec)

mysql> alter table bbs_user2 add primary key(id);    // alter修改表属性   add添加
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc bbs_user2;
+----------+------------------+------+-----+---------+-------+
| Field    | Type             | Null | Key | Default | Extra |
+----------+------------------+------+-----+---------+-------+
| id       | int(10) unsigned | NO   | PRI | NULL    |       |
| username | varchar(32)      | NO   |     | NULL    |       |
+----------+------------------+------+-----+---------+-------+
2 rows in set (0.03 sec)

mysql> alter table bbs_user2 modify id int unsigned not null auto_increment;
Query OK, 0 rows affected (0.02 sec)                 //modify 修改字段属性
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc bbs_user2;
+----------+------------------+------+-----+---------+----------------+
| Field    | Type             | Null | Key | Default | Extra          |
+----------+------------------+------+-----+---------+----------------+
| id       | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(32)      | NO   |     | NULL    |                |
+----------+------------------+------+-----+---------+----------------+
2 rows in set (0.03 sec)

mysql> alter table bbs_user2 change id uid unsigned not null auto_increment prim
ary key ;                               
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that
corresponds to your MySQL server version for the right syntax to use near 'unsig
ned not null auto_increment primary key' at line 1
mysql> alter table bbs_user2 change id uid int unsigned not null auto_increment
primary key ;                                             //报错 以有创建主键 再次创建
ERROR 1068 (42000): Multiple primary key defined          //去掉primary key ；
mysql> alter table bbs_user2 change id uid int unsigned not null auto_increment;

Query OK, 0 rows affected (0.01 sec)                      //change 修改 
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc bbs_user2;
+----------+------------------+------+-----+---------+----------------+
| Field    | Type             | Null | Key | Default | Extra          |
+----------+------------------+------+-----+---------+----------------+
| uid      | int(10) unsigned | NO   | PRI | NULL    | auto_increment |
| username | varchar(32)      | NO   |     | NULL    |                |
+----------+------------------+------+-----+---------+----------------+
2 rows in set (0.02 sec)







 


                                        注意：change  可以改变字段名称
                                              modify  可以修改字段类型