oracle comment命令用法示例

oracle中用comment on命令给表或字段加以说明，语法如下：

代码如下:
COMMENT ON
{ TABLE [ schema. ]
{ table | view }
| COLUMN [ schema. ]
{ table. | view. | materialized_view. } column
| OPERATOR [ schema. ] operator
| INDEXTYPE [ schema. ] indextype
| MATERIALIZED VIEW materialized_view
}
IS 'text' ;

用法如下：

1.对表的说明

代码如下:
comment on table table_name is 'comments_on_tab_information';

2.对表中列的说明

代码如下:
comment on column table.column_name is 'comments_on_col_information';

3.查看表的说明

代码如下:
SQL> select * from user_tab_comments where TABLE_NAME='EMPLOYEES';

代码如下:
TABLE_NAME TABLE_TYPE COMMENTS
------------------------------ ----------- ----------
EMPLOYEES TABLE 员工表

SQL> select * from user_tab_comments where comments is not null;

TABLE_NAME TABLE_TYPE COMMENTS
------------------------------ ----------- --------------------------
EMPLOYEES TABLE 员工表

4.查看表中列的说明

代码如下:
SQL> select * from user_col_comments where TABLE_NAME='EMPLOYEES';

代码如下:
TABLE_NAME COLUMN_NAME COMMENTS
------------------------------ ------------------------------ ------------
EMPLOYEES EMPLOYEE_ID
EMPLOYEES MANAGER_ID
EMPLOYEES FIRST_NAME
EMPLOYEES LAST_NAME
EMPLOYEES TITLE
EMPLOYEES SALARY 员工薪水

SQL> select * from user_col_comments where comments is not null;

TABLE_NAME COLUMN_NAME COMMENTS
------------------------------ ------------------------------ -------------
EMPLOYEES SALARY 员工薪水

5.我们也可以从下面这些视图中查看表级和列级说明：

代码如下:
ALL_COL_COMMENTS
USER_COL_COMMENTS
ALL_TAB_COMMENTS
USER_TAB_COMMENTS

6.删除表级说明，也就是将其置为空

代码如下:
SQL> comment on table employees is '';
Comment added

SQL> select * from user_tab_comments where TABLE_NAME='EMPLOYEES';

TABLE_NAME TABLE_TYPE COMMENTS
------------------------------ ----------- -------------
EMPLOYEES TABLE

7.删除列级说明，也是将其置为空

代码如下:
SQL> comment on column employees.salary is '';
Comment added

SQL> select * from user_col_comments where TABLE_NAME='EMPLOYEES';

TABLE_NAME COLUMN_NAME COMMENTS
------------------------------ ------------------------------ -------------
EMPLOYEES EMPLOYEE_ID
EMPLOYEES MANAGER_ID
EMPLOYEES FIRST_NAME
EMPLOYEES LAST_NAME
EMPLOYEES TITLE
EMPLOYEES SALARY

添加字段的语法：alter table tablename add (column datatype [default value][null/not null],….);修改字段的语法：alter ...