修改数据库中一个表的字段长度，怎么做

1、通过图形界面 *** 作，在左侧依次选择objects-tables，右键单击要修改的表名，选中‘Edit’-column，可以直接修改；

2、使用DDL语句：alter table 表名 modify 字段名(字符类型(长度))

例如：

alter table emp modify ename(varchar2(32))

gbase如何增加多个字段

GBase 8a数据库集群，目前不支持字段的定义修改，除了varchar类型可以增加长度，其它的类型或属性均不可以，需要重建一个字段过渡一下。

varchar类型增加长度

请一定保留原有的附加属性，包括not null, default 等。否则change时会报错。单独修改注释等，请用modify功能。

gbase>desc t2

+-------+-------------+------+-----+---------+-------+

| Field | Type| Null | Key | Default | Extra |

+-------+-------------+------+-----+---------+-------+

| id| int(11) | YES | | NULL| |

| name | varchar(20) | YES | | NULL| |

+-------+-------------+------+-----+---------+-------+

2 rows in set (Elapsed: 00:00:00.00)

gbase>alter table t2 change name name varchar(30)

Query OK, 0 rows affected, 1 warning (Elapsed: 00:00:00.98)

Records: 0 Duplicates: 0 Warnings: 0

gbase>desc t2

+-------+-------------+------+-----+---------+-------+

| Field | Type| Null | Key | Default | Extra |

+-------+-------------+------+-----+---------+-------+

| id| int(11) | YES | | NULL| |

| name | varchar(30) | YES | | NULL| |

+-------+-------------+------+-----+---------+-------+

2 rows in set (Elapsed: 00:00:00.00)

gbase>

其它类型变动

其它类型只有先建一个新的字段，然后把数据update过去，然后把老的删除，把新的change成老的字段名。

如下例子，把value bigint, 改成 value int.

gbase>desc t1

+-------+------------+------+-----+---------+-------+

| Field | Type | Null | Key | Default | Extra |

+-------+------------+------+-----+---------+-------+

| id| int(11)| YES | MUL | NULL| |

| value | bigint(20) | YES | | NULL| |

| birth | datetime | YES | | NULL| |

+-------+------------+------+-----+---------+-------+

3 rows in set (Elapsed: 00:00:00.00)

gbase>alter table t1 add column value2 int after value

Query OK, 4 rows affected (Elapsed: 00:00:00.65)

Records: 4 Duplicates: 4 Warnings: 0

gbase>desc t1

+--------+------------+------+-----+---------+-------+

| Field | Type | Null | Key | Default | Extra |

+--------+------------+------+-----+---------+-------+

| id | int(11)| YES | MUL | NULL| |

| value | bigint(20) | YES | | NULL| |

| value2 | int(11)| YES | | NULL| |

| birth | datetime | YES | | NULL| |

+--------+------------+------+-----+---------+-------+

4 rows in set (Elapsed: 00:00:00.00)

gbase>update t1 set value2=value

Query OK, 4 rows affected (Elapsed: 00:00:00.28)

Rows matched: 4 Changed: 4 Warnings: 0

gbase>alter table t1 drop value

Query OK, 4 rows affected (Elapsed: 00:00:00.42)

Records: 4 Duplicates: 4 Warnings: 0

gbase>desc t1

+--------+----------+------+-----+---------+-------+

| Field | Type | Null | Key | Default | Extra |

+--------+----------+------+-----+---------+-------+

| id | int(11) | YES | MUL | NULL| |

| value2 | int(11) | YES | | NULL| |

| birth | datetime | YES | | NULL| |

+--------+----------+------+-----+---------+-------+

3 rows in set (Elapsed: 00:00:00.00)

gbase>alter table t1 change value2 value int

Query OK, 0 rows affected (Elapsed: 00:00:00.21)

Records: 0 Duplicates: 0 Warnings: 0

gbase>desc t1

+-------+----------+------+-----+---------+-------+

| Field | Type | Null | Key | Default | Extra |

+-------+----------+------+-----+---------+-------+

| id| int(11) | YES | MUL | NULL| |

| value | int(11) | YES | | NULL| |

| birth | datetime | YES | | NULL| |

+-------+----------+------+-----+---------+-------+

3 rows in set (Elapsed: 00:00:00.01)

本文的功能经常用到，虽然系统可以修改字段类型，但还是建议在设计阶段控制好，特别是避免从高精度，高长度更改为低精度，低长度的情况。 特别是varchar并不是实际占用空间。

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