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bitsCN.com mysql中的auto_increment的问题 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: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。 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。[html]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> delete from test1 where id in (10,11,9);  Query OK, 3 rows affected (0.03 sec)    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=12 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.    Oracle is a registered trademark of Oracle Corporation and/or its  affiliates. Other names may be trademarks of their respective  owners.    Type 'help;' or '/h' for help. Type '/c' to clear the current input statement.    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    Database changed  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=9 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。[html]mysql> select last_insert_id();  +------------------+  | last_insert_id() |  +------------------+  |                0 |  +------------------+  1 row in set (0.00 sec)     作者 chen861201 bitsCN.com

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