- 重新定义字段的属性，去除约束关键字既可删除约束
删除默认约束
- 删除默认约束可以使用语句有 ALTER ALTER … DROP DEFAULT，MODIFY 语句，CHANGE 语句
语法
ALTER TABLE <tbl_name>
ALTER <colum_name> DROP DEFAULT;
ALTER TABLE <tbl_name>
MODIFY <colum_name> column_definition;
ALTER TABLE <tbl_name>
MODIFY <colum_name> column_definition DEFAULT NULL;
ALTER TABLE <tbl_name>
CHANGE <colum_name> <new_colum_name> column_definition DEFAULT NULL
|
参数 |
描述 |
|
<tbl_name> |
数据表名 |
|
<column_name> <new_column_name> |
数据表列名 |
|
column_definition |
数据列定义 |
示例
ALTER TABLE student
ALTER id DROP DEFAULT;
Query OK, 0 rows affected (0.15 sec)
Records: 0 Duplicates: 0 Warnings: 0
示例
ALTER TABLE student
MOFIFY name VARCHAR(20);
Query OK, 0 rows affected (0.15 sec)
Records: 0 Duplicates: 0 Warnings: 0
示例
ALTER TABLE student
MOFIFY name VARCHAR(20) DEFAULT NULL;
Query OK, 0 rows affected (0.15 sec)
Records: 0 Duplicates: 0 Warnings: 0
示例
ALTER TABLE student
CHANGE name name VARCHAR(20) DEFAULT NULL;
Query OK, 0 rows affected (0.15 sec)
Records: 0 Duplicates: 0 Warnings: 0
删除非空约束
- 删除非空约束可以使用语句有 MODIFY 语句，CHANGE 语句
语法
ALTER TABLE <tbl_name>
MODIFY <colum_name> column_definition NULL;
ALTER TABLE <tbl_name>
MODIFY <colum_name> column_definition;
ALTER TABLE <tbl_name>
CHANGE <colum_name> <new_colum_name> column_definition NULL;
ALTER TABLE <tbl_name>
CHANGE <colum_name> <new_colum_name> column_definition;
|
参数 |
描述 |
|
<tbl_name> |
数据表名 |
|
<column_name> <new_column_name> |
数据表列名 |
|
column_definition |
数据列定义 |
示例
ALTER TABLE student
MOFIFY name VARCHAR(20);
ALTER TABLE student
MOFIFY name VARCHAR(20) NULL;
Query OK, 0 rows affected (0.15 sec)
Records: 0 Duplicates: 0 Warnings: 0
示例
ALTER TABLE student
CHANGE name name VARCHAR(20);
ALTER TABLE student
CHANGE name name VARCHAR(20) NULL;
Query OK, 0 rows affected (0.15 sec)
Records: 0 Duplicates: 0 Warnings: 0
删除自增约束
- 删除自增约束可以使用的语句有 MODIFY 语句，CHANGE 语句
语法
ALTER TABLE <tbl_name>
MODIFY <colum_name> column_definition;
ALTER TABLE <tbl_name>
CHANGE <colum_name> <new_colum_name> column_definition;
|
参数 |
描述 |
|
<tbl_name> |
数据表名 |
|
<column_name> <new_column_name> |
数据表列名 |
|
column_definition |
数据列定义 |
示例
ALTER TABLE student
MOFIFY name VARCHAR(20);
Query OK, 0 rows affected (0.15 sec)
Records: 0 Duplicates: 0 Warnings: 0
示例
ALTER TABLE student
CHANGE name name VARCHAR(20);
Query OK, 0 rows affected (0.15 sec)
Records: 0 Duplicates: 0 Warnings: 0