如何理解mysql自增长列

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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。