MySQL表怎么创建自增字段
本篇内容介绍了"MySQL表怎么创建自增字段"的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！
注：如果使用新的自增互斥方式，对于replication应该避免使用INSERT ... ON DUPLICATE KEY UPDATE语句。
设置新自增互斥方式：通过配置选项：innodb_autoinc_lock_mode：调整锁策略：
innodb_autoinc_lock_mode = 0 ("traditional" lock mode：全部使用表锁)
innodb_autoinc_lock_mode = 1 (默认)("consecutive" lock mode：可预判行数时使用新方式，不可时使用表锁)
innodb_autoinc_lock_mode = 2 ("interleaved" lock mode：全部使用新方式，不安全，不适合replication)
##创建自增字段
方法1、创建:
mysql> create table c(id int auto_increment,name varchar(20),primary key(id));
Query OK, 0 rows affected (0.52 sec)
mysql> desc c;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+----------------+
2 rows in set (0.03 sec)
方法2、修改:
mysql> create table cc (id int,name varchar(20));
Query OK, 0 rows affected (0.42 sec)
mysql> alter table cc change id id int primary key auto_increment;
Query OK, 0 rows affected (1.12 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> desc cc;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+----------------+
2 rows in set (0.11 sec)
mysql> insert into cc(id,name) values(1,'a'),(NULL,'b'),(NULL,'c'),(5,'d');
mysql> select * from cc;
+----+------+
| id | name |
+----+------+
| 1 | a |
| 2 | b |
| 3 | c |
| 5 | d |
+----+------+
4 rows in set (0.00 sec)
注:只有int类型且为primary key 才可以使用auto_increment.
##对存在记录的表的列修改为自增列
mysql> create table ccc (id int,name varchar(20));
Query OK, 0 rows affected (0.27 sec)
mysql> insert into ccc(id,name) values(1,'a'),(NULL,'b'),(NULL,'c'),(5,'d');
Query OK, 4 rows affected (0.53 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> select * from ccc;
+------+------+
| id | name |
+------+------+
| 1 | a |
| NULL | b |
| NULL | c |
| 5 | d |
+------+------+
4 rows in set (0.00 sec)
mysql> alter table ccc change id id int primary key auto_increment;
Query OK, 4 rows affected (1.04 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> desc ccc;
+-------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+----------------+
2 rows in set (0.01 sec)
mysql> insert into ccc(id,name) values(1,'a'),(NULL,'b'),(NULL,'c'),(5,'d');
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'
mysql> insert into ccc(id,name) values(6,'aa'),(NULL,'ab'),(NULL,'ac'),(10,'ad')
;
Query OK, 4 rows affected (0.07 sec)
Records: 4 Duplicates: 0 Warnings: 0
mysql> select * from ccc;
+----+------+
| id | name |
+----+------+
| 1 | a |
| 2 | b |
| 3 | c |
| 5 | d |
| 6 | aa |
| 7 | ab |
| 8 | ac |
| 10 | ad |
+----+------+
8 rows in set (0.00 sec)
mysql>
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