oracle comment on的用法

oracle中用comment
on命令给表或字段加以说明，语法如下：
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
comment on table table_name is  'comments_on_tab_information';
2.对表中列的说明
comment on column table.column_name is 'comments_on_col_information';
3.查看表的说明
SQL> select * from user_tab_comments where TABLE_NAME='EMPLOYEES';
SQL> select * from user_tab_comments where comments is not null;

4.查看表中列的说明
SQL> select * from user_col_comments where TABLE_NAME='EMPLOYEES';
SQL> select * from user_col_comments where comments is not null;

5.我们也可以从下面这些视图中查看表级和列级说明：
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
SQL> comment on table employees is '';
SQL> select * from user_tab_comments where TABLE_NAME='EMPLOYEES';

7.删除列级说明，也是将其置为空
SQL> comment on column employees.salary is '';
SQL> selec *  from user_col_comments where TABLE_NAME='EMPLOYEES';