ALTER TABLE tbl_name ADD PRIMARY KEY (index_col_name,…);
/*
创建一个qq表，将qq_id设为主键，且没有对其进行NOT NULl约束
*/
create table qq(
qq_id int(10),
nick_name varchar(255) not null,
primary key (qq_id))
/*
插入一条数据，将qq号设为10000(咱也幻想一下)，昵称设为"simaopig"
*/
INSERT INTO qq( qq_id, nick_name )
VALUES (
'10000', 'simaopig');

/*
创建防火墙表，将host 和port组合设为主键，注意我没有将port设NOT NULL约束
*/
create table firewall(
host varchar(11) not null,
port smallint(4),
access enum('deny', 'allow') not null,
primary key (host,port))
/*
插入一条新的记录，没有啥问题
1 row(s) inserted.
*/
INSERT INTO firewall (
host ,
port ,
access)
VALUES (
'202.65.3.87', '21', 'deny');

mysql> create database ssh2; 

Query OK, 1 row affected (0.04 sec) 

mysql> use ssh2; 

Database changed 

mysql> create table user( 
  -> id integer primary key, 
  -> firstname varchar(200) not null, 
  -> lastname varchar(200) not null, 
  -> age integer 
  -> ); 

Query OK, 0 rows affected (0.46 sec) 

mysql> alter table user modify id integer auto_increment ; 

Query OK, 1 row affected (0.28 sec) 
Records: 1 Duplicates: 0 Warnings: 0 

mysql> alter table user modify id integer auto_increment ; 

Query OK, 0 rows affected (0.39 sec) 
Records: 0 Duplicates: 0 Warnings: 0 

mysql> alter table user modify id integer default '1'; 

Query OK, 0 rows affected (0.16 sec) 
Records: 0 Duplicates: 0 Warnings: 0 

mysql> alter table user modify id integer auto_increment ; 

Query OK, 1 row affected (0.28 sec) 
Records: 1 Duplicates: 0 Warnings: 0 

mysql> alter table user add createtime timestamp default current_timestamp; 

Query OK, 2 rows affected (0.17 sec) 
Records: 2 Duplicates: 0 Warnings: 0 

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

Query OK, 0 rows affected (0.10 sec) 

 
mysql> insert into dd(name) values ('fhihgifds');

Query OK, 1 row affected (0.14 sec) 

mysql> insert into dd(name) values ('steven'); 

Query OK, 1 row affected (0.08 sec) 

mysql> select * from dd; 

+----+-----------+---------------------+ 
| id | name   | time        | 
+----+-----------+---------------------+ 
| 1 | fhihgifds | 2011-03-27 01:58:46 | 
| 2 | steven  | 2011-03-27 01:59:35 | 
+----+-----------+---------------------+ 
2 rows in set (0.08 sec) 

mysql> insert into dd(name) values ('anthony'); 

Query OK, 1 row affected (0.09 sec) 

mysql> select * from dd; 

+----+-----------+---------------------+ 
| id | name   | time        | 
+----+-----------+---------------------+ 
| 1 | fhihgifds | 2011-03-27 01:58:46 | 
| 2 | steven  | 2011-03-27 01:59:35 | 
| 3 | anthony  | 2011-03-27 02:00:07 | 
+----+-----------+---------------------+ 
3 rows in set (0.00 sec)