mysql> create table ss(id int unsigned not null primary key auto_increment, user_name varchar(15) not null);
Query OK, 0 rows affected (0.00 sec)
mysql>
mysql> insert into ss(id,user_name) values(1, 'jojo');
Query OK, 1 row affected (0.00 sec)
mysql> insert into ss(id,user_name) values(37, 'liz');
Query OK, 1 row affected (0.00 sec)
mysql>
mysql> select id, user_name from ss;
+----+-----------+
| id | user_name |
+----+-----------+
| 1 | jojo |
| 37 | liz |
+----+-----------+
2 rows in set (0.00 sec)
mysql>
===========================================
mysql> create table uu(id int unsigned not null primary key auto_increment, user_name varchar(15) not null) auto_increment=100;
Query OK, 0 rows affected (0.01 sec)
mysql>
=================================
mysql> insert into uu(id,user_name) values(1, 'jojo');
Query OK, 1 row affected (0.00 sec)
mysql> insert into uu(id,user_name) values(37, 'liz');
Query OK, 1 row affected (0.00 sec)
mysql> select id, user_name from uu;
+----+-----------+
| id | user_name |
+----+-----------+
| 1 | jojo |
| 37 | liz |
+----+-----------+
2 rows in set (0.00 sec)
mysql> desc uu
-> ;
+-----------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------+------------------+------+-----+---------+----------------+
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
| user_name | varchar(15) | NO | | NULL | |
+-----------+------------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
mysql>
当插入记录时，没有为AUTO_INCREMENT明确指定值，则等同插入NULL值
把0插入AUTO_INCREMENT数据列的效果与插入NULL值一样。但不建议这样做，还是以插入NULL值为好。
如果把一个NULL插入到一个AUTO_INCREMENT数据列里去，MySQL将自动生成下一个序 列编号。编号从1开始，并1为基数递增。
因为上面指定了auto_increment=100，所以编号从100开始，递增1。
mysql> insert into uu(user_name) values('Nicky');
Query OK, 1 row affected (0.00 sec)
mysql> select id, user_name from uu;
+-----+-----------+
| id | user_name |
+-----+-----------+
| 1 | jojo |
| 37 | liz |
| 100 | Nicky |
+-----+-----------+
3 rows in set (0.00 sec)
mysql> insert into uu(user_name) values('Lucy');
Query OK, 1 row affected (0.00 sec)
mysql> select id, user_name from uu;
+-----+-----------+
| id | user_name |
+-----+-----------+
| 1 | jojo |
| 37 | liz |
| 100 | Nicky |
| 101 | Lucy |
+-----+-----------+
4 rows in set (0.00 sec)
mysql>
mysql建表设置两个默认CURRENT_TIMESTAMP的技巧
create table t (ids int not null auto_increment primary key, name1 varchar(20), t1 timestamp default '0000-00-00 00:00:00', t2 timestamp default now() on update now())
insert into t(name1) values ('tom')
insert into t(name1,t1, t2) values ('jerry', null, null)
