《MysqL数据库解析MysqL中的auto_increment的问题》要点：
本文介绍了MysqL数据库解析MysqL中的auto_increment的问题，希望对您有用。如果有疑问，可以联系我们。
MysqL入门今儿在逛论坛时,发现一个看似很简单的问题,却引起了大家的广泛关注：
这是一道很早的面试题：
一张表,里面有ID自增主键,当insert了17条记录之后,删除了第15,16,17条记录,再把MysqL重启,再insert一条记录,这条记录的ID是18还是15 .
答案：
MysqL入门如果表的类型是MyISAM,那么是18.
因为MyISAM表会把自增主键的最大ID记录到数据文件里,重启MysqL自增主键的最大ID也不会丢失.
如果表的类型是InnoDB,那么是15.
InnoDB表只是把自增主键的最大ID记录到内存中,所以重启数据库或者是对表进行OPTIMIZE操作,都会导致最大ID丢失.
MysqL入门自己也做了个实验,结果证实了上面的说法.真是惭愧啊,看似简单的问题自己也打错了.
MysqL> select * from test1;
+----+-----------+
| id | name |
+----+-----------+
| 1 | 陈兵辉 |
| 2 | chen |
| 3 | chen |
| 4 | chen |
| 5 | chen |
| 6 | chen |
| 7 | chen |
| 8 | chen |
| 9 | chen |
| 10 | chen |
| 11 | chen |
+----+-----------+
11 rows in set (0.00 sec)
MysqL入门MysqL> delete from test1 where id in (10,11,9);
Query OK,3 rows affected (0.03 sec)
MysqL入门MysqL> show create table test1;
CREATE TABLE `test1` (
`id` int(11) NOT NULL auto_increment,
`name` varchar(10) default NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=<SPAN style="COLOR: #ff0000">12</SPAN> DEFAULT CHARSET=utf8 |
MysqL> exit;
Bye
[root@fsailing1 ~]# service MysqLd restart
停止 MysqL： [确定]
启动 MysqL： [确定]
[root@fsailing1 ~]# MysqL -uroot -p
Enter password:
Welcome to the MysqL monitor. Commands end with ; or \g.
Your MysqL connection id is 2
Server version: 5.0.95 Source distribution
Copyright (c) 2000,2011,Oracle and/or its affiliates. All rights reserved.
MysqL入门Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
MysqL入门Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
MysqL入门MysqL> use test;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
MysqL入门Database changed
MysqL> show create table test1;
| CREATE TABLE `test1` (
`id` int(11) NOT NULL auto_increment,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=<SPAN style="COLOR: #ff0000">9</SPAN> DEFAULT CHARSET=utf8 |
2,另外还有一个就是获取当前数据库表的自增字段数.
MysqL> select last_insert_id();
+------------------+
| last_insert_id() |
+------------------+
| 0 |
+------------------+
1 row in set (0.00 sec)