如何理解mysql自增长列
发布时间:2021-12-18 13:29:55 所属栏目:MySql教程 来源:互联网
导读:本篇文章给大家分享的是有关如何理解mysql自增长列,小编觉得挺实用的,因此分享给大家学习,希望大家阅读完这篇文章后可以有所收获,话不多说,跟着小编一起来看看吧。 自增长列必须是索引列,否则无法创建成功表,对myisma和innodb都一样 (localhost@test
本篇文章给大家分享的是有关如何理解mysql自增长列,小编觉得挺实用的,因此分享给大家学习,希望大家阅读完这篇文章后可以有所收获,话不多说,跟着小编一起来看看吧。 自增长列必须是索引列,否则无法创建成功表,对myisma和innodb都一样 (localhost@testdb)[root]> create table test5 (id int auto_increment,name varchar(10)) engine=innodb; ERROR 1075 (42000): (localhost@testdb)[root]> (localhost@testdb)[root]> (localhost@testdb)[root]> create table test5 (id int auto_increment,name varchar(10),index(id)) engine=innodb; Query OK, 0 rows affected (0.01 sec) (localhost@testdb)[root]> create table test5 (id int auto_increment,name varchar(10)) engine=myisam; ERROR 1075 (42000): (localhost@testdb)[root]> create table test5 (id int auto_increment,name varchar(10),index(id)) engine=myisam; Query OK, 0 rows affected (0.00 sec) (localhost@testdb)[root]> (localhost@testdb)[root]> (localhost@testdb)[root]> 创建成功后id列没有插入数据,但是可以自动增长 (localhost@testdb)[root]> insert into test5(name) values('aa'),('bb'),('cc'); Query OK, 3 rows affected (0.00 sec) (localhost@testdb)[root]> select * from test5; +----+------+ | id | name | +----+------+ | 1 | aa | | 2 | bb | | 3 | cc | +----+------+ 3 rows in set (0.00 sec) 索引 (localhost@testdb)[root]> (localhost@testdb)[root]> show index from test5; +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ | test5 | 1 | id | 1 | id | A | NULL | NULL | NULL | | BTREE | | | +-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+ 1 row in set (0.00 sec) 删除表里的数据,在插入数据id列会依据原来的值继续增长 (localhost@testdb)[root]> delete from test5; Query OK, 3 rows affected (0.00 sec) (localhost@testdb)[root]> (localhost@testdb)[root]> (localhost@testdb)[root]> select * from test5; Empty set (0.00 sec) (localhost@testdb)[root]> insert into test5(name) values('aa'),('bb'),('cc'); Query OK, 3 rows affected (0.00 sec) (localhost@testdb)[root]> select * from test5; +----+------+ | id | name | +----+------+ | 4 | aa | | 5 | bb | | 6 | cc | +----+------+ 3 rows in set (0.00 sec) truncate 表里的数据后在插入数据,id列会从1开始增长。 (localhost@testdb)[root]> truncate table test5; Query OK, 0 rows affected (0.00 sec) (localhost@testdb)[root]> select * from test5; Empty set (0.00 sec) (localhost@testdb)[root]> insert into test5(name) values('aa'),('bb'),('cc'); Query OK, 3 rows affected (0.00 sec) (localhost@testdb)[root]> select * from test5; +----+------+ | id | name | +----+------+ | 1 | aa | | 2 | bb | | 3 | cc | +----+------+ 3 rows in set (0.00 sec) (localhost@testdb)[root]> 对于复合索引的自增长列 myisam引擎的自增长列,在索引中是非前导列可以创建成功 innodb引擎的自增长列,在索引中必须是前导列,表才能创建成功 (localhost@testdb)[root]> create table test4 (id1 int auto_increment,id2 int,name varchar(10),index(id2,id1)) engine=myisam; Query OK, 0 rows affected (0.00 sec) (localhost@testdb)[root]> (localhost@testdb)[root]> (localhost@testdb)[root]> drop table test4; Query OK, 0 rows affected (0.00 sec) (localhost@testdb)[root]> create table test4 (id1 int auto_increment,id2 int,name varchar(10),index(id2,id1)) engine=innodb; ERROR 1075 (42000): (localhost@testdb)[root]> (localhost@testdb)[root]> (localhost@testdb)[root]> create table test4 (id1 int auto_increment,id2 int,name varchar(10),index(id1,id2)) engine=innodb; Query OK, 0 rows affected (0.01 sec) (localhost@testdb)[root]> (localhost@testdb)[root]> (localhost@testdb)[root]> 以上就是如何理解mysql自增长列,小编相信有部分知识点可能是我们日常工作会见到或用到的。希望你能通过这篇文章学到更多知识。更多详情敬请关注亿速云行业资讯频道。 (编辑:应用网_丽江站长网) 【声明】本站内容均来自网络,其相关言论仅代表作者个人观点,不代表本站立场。若无意侵犯到您的权利,请及时与联系站长删除相关内容! |