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';

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
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                      
EMPLO