ALTER SEQUENCE
语法说明
ALTER SEQUENCE
用于修改现有序列。
语法结构
> ALTER SEQUENCE [ IF EXISTS ] SEQUENCE_NAME
[ AS data_type ]
[ INCREMENT [ BY ] increment ]
[ MINVALUE minvalue] [ MAXVALUE maxvalue]
[ START [ WITH ] start ] [ [ NO ] CYCLE ]
语法释义
-
[ IF EXISTS ]
:可选的子句,表示如果指定的序列不存在,也不会引发错误。如果使用了此子句,系统将检查序列是否存在,如果不存在,将忽略修改请求。 -
SEQUENCE_NAME
:要修改的序列的名称。 -
[ AS data_type ]
:可选子句,它允许您为序列指定数据类型。通常,序列的数据类型是整数。 -
[ INCREMENT [ BY ] increment ]
:这是指定序列的增量值。序列的增量值是在每次递增或递减时要添加到当前值的数量。如果未指定增量值,通常默认为 1。 -
[ MINVALUE minvalue ]
:这是序列的最小值,它指定了序列允许的最小值。如果指定了最小值,序列的当前值不能低于此值。 -
[ MAXVALUE maxvalue ]
:这是序列的最大值,它指定了序列允许的最大值。如果指定了最大值,序列的当前值不能超过此值。 -
[ START [ WITH ] start ]
:这是序列的起始值,它指定序列的初始值。如果未指定起始值,通常默认为 1。 -
[ [ NO ] CYCLE ]
:可选子句,用于指定是否循环使用序列值。如果指定了NO CYCLE
,则在达到最大值或最小值后,序列将停止递增或递减。如果未指定此子句,通常默认为不循环。
示例
-- 创建一个名为 alter_seq_01 的序列,将序列的增量设置为 2,设置序列的最小值为 30,最大值为 100,并启用循环
create sequence alter_seq_01 as smallint increment by 2 minvalue 30 maxvalue 100 cycle;
mysql> show sequences;
+--------------+-----------+
| Names | Data Type |
+--------------+-----------+
| alter_seq_01 | SMALLINT |
+--------------+-----------+
1 row in set (0.00 sec)
mysql> alter sequence alter_seq_01 as bigint;
Query OK, 0 rows affected (0.01 sec)
mysql> show sequences;
+--------------+-----------+
| Names | Data Type |
+--------------+-----------+
| alter_seq_01 | BIGINT |
+--------------+-----------+
1 row in set (0.00 sec)
-- 取消序列 alter_seq_01 的循环
mysql> alter sequence alter_seq_01 no cycle;
Query OK, 0 rows affected (0.01 sec)
mysql> select nextval('alter_seq_01'),currval('alter_seq_01');
+-----------------------+-----------------------+
| nextval(alter_seq_01) | currval(alter_seq_01) |
+-----------------------+-----------------------+
| 30 | 30 |
+-----------------------+-----------------------+
1 row in set (0.01 sec)
mysql> select nextval('alter_seq_01'),currval('alter_seq_01');
+-----------------------+-----------------------+
| nextval(alter_seq_01) | currval(alter_seq_01) |
+-----------------------+-----------------------+
| 32 | 32 |
+-----------------------+-----------------------+
1 row in set (0.00 sec)
-- 将序列 alter_seq_01 的起始值设置为 40
mysql> alter sequence alter_seq_01 start with 40;
Query OK, 0 rows affected (0.01 sec)
mysql> select nextval('alter_seq_01'),currval('alter_seq_01');
+-----------------------+-----------------------+
| nextval(alter_seq_01) | currval(alter_seq_01) |
+-----------------------+-----------------------+
| 40 | 40 |
+-----------------------+-----------------------+
1 row in set (0.01 sec)
mysql> select nextval('alter_seq_01'),currval('alter_seq_01');
+-----------------------+-----------------------+
| nextval(alter_seq_01) | currval(alter_seq_01) |
+-----------------------+-----------------------+
| 42 | 42 |
+-----------------------+-----------------------+
1 row in set (0.00 sec)
-- 将序列 alter_seq_01 的增量值设置为 3
mysql> alter sequence alter_seq_01 increment by 3;
Query OK, 0 rows affected (0.01 sec)
mysql> select nextval('alter_seq_01'),currval('alter_seq_01');
+-----------------------+-----------------------+
| nextval(alter_seq_01) | currval(alter_seq_01) |
+-----------------------+-----------------------+
| 40 | 40 |
+-----------------------+-----------------------+
1 row in set (0.00 sec)
mysql> select nextval('alter_seq_01'),currval('alter_seq_01');
+-----------------------+-----------------------+
| nextval(alter_seq_01) | currval(alter_seq_01) |
+-----------------------+-----------------------+
| 43 | 43 |
+-----------------------+-----------------------+
1 row in set (0.00 sec)