mysql> alter table user modify id integer auto_increment ;
query ok, 1 row affected (0.28 sec)
records: 1 duplicates: 0 warnings: 0
給主鍵增加乙個自增的功能：
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)
這樣，上面的user表裡面的主鍵，id可以自增了。
給上面的主鍵id增加預設值和自增功能：
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設定主鍵不能為空，還要自動增長（這裡沒有設定預設值，但是預設是1，從1開始增長。），還要得到系統預設日期：
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)
mysql>
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