mysql中的auto_increment的问题_MySQL
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2022-05-24 20:22:29
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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
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