ALTER SEQUENCE

语法说明

ALTER SEQUENCE 用于修改现有序列。

语法结构

1. > ALTER SEQUENCE [ IF EXISTS ] SEQUENCE_NAME
2. [ AS data_type ]
3. [ INCREMENT [ BY ] increment ]
4. [ MINVALUE minvalue] [ MAXVALUE maxvalue]
5. [ 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

1. -- 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
2. create sequence alter_seq_01 as smallint increment by 2 minvalue 30 maxvalue 100 cycle;

4. mysql> show sequences;
5. +--------------+-----------+
6. | Names        | Data Type |
7. +--------------+-----------+
8. | alter_seq_01 | SMALLINT  |
9. +--------------+-----------+
10. 1 row in set (0.00 sec)

12. mysql> alter sequence alter_seq_01 as bigint;
13. Query OK, 0 rows affected (0.01 sec)

15. mysql> show sequences;
16. +--------------+-----------+
17. | Names        | Data Type |
18. +--------------+-----------+
19. | alter_seq_01 | BIGINT    |
20. +--------------+-----------+
21. 1 row in set (0.00 sec)

23. -- 取消序列 alter_seq_01 的循环
24. mysql> alter sequence alter_seq_01 no cycle;
25. Query OK, 0 rows affected (0.01 sec)

27. mysql> select nextval('alter_seq_01'),currval('alter_seq_01');
28. +-----------------------+-----------------------+
29. | nextval(alter_seq_01) | currval(alter_seq_01) |
30. +-----------------------+-----------------------+
31. | 30                    | 30                    |
32. +-----------------------+-----------------------+
33. 1 row in set (0.01 sec)

35. mysql> select nextval('alter_seq_01'),currval('alter_seq_01');
36. +-----------------------+-----------------------+
37. | nextval(alter_seq_01) | currval(alter_seq_01) |
38. +-----------------------+-----------------------+
39. | 32                    | 32                    |
40. +-----------------------+-----------------------+
41. 1 row in set (0.00 sec)

43. -- 将序列 alter_seq_01 的起始值设置为 40
44. mysql> alter sequence alter_seq_01 start with 40;
45. Query OK, 0 rows affected (0.01 sec)

47. mysql> select nextval('alter_seq_01'),currval('alter_seq_01');
48. +-----------------------+-----------------------+
49. | nextval(alter_seq_01) | currval(alter_seq_01) |
50. +-----------------------+-----------------------+
51. | 40                    | 40                    |
52. +-----------------------+-----------------------+
53. 1 row in set (0.01 sec)

55. mysql> select nextval('alter_seq_01'),currval('alter_seq_01');
56. +-----------------------+-----------------------+
57. | nextval(alter_seq_01) | currval(alter_seq_01) |
58. +-----------------------+-----------------------+
59. | 42                    | 42                    |
60. +-----------------------+-----------------------+
61. 1 row in set (0.00 sec)

63. -- 将序列 alter_seq_01 的增量值设置为 3
64. mysql> alter sequence alter_seq_01 increment by 3;
65. Query OK, 0 rows affected (0.01 sec)

67. mysql> select nextval('alter_seq_01'),currval('alter_seq_01');
68. +-----------------------+-----------------------+
69. | nextval(alter_seq_01) | currval(alter_seq_01) |
70. +-----------------------+-----------------------+
71. | 40                    | 40                    |
72. +-----------------------+-----------------------+
73. 1 row in set (0.00 sec)

75. mysql> select nextval('alter_seq_01'),currval('alter_seq_01');
76. +-----------------------+-----------------------+
77. | nextval(alter_seq_01) | currval(alter_seq_01) |
78. +-----------------------+-----------------------+
79. | 43                    | 43                    |
80. +-----------------------+-----------------------+
81. 1 row in set (0.00 sec)