oracle comment on usage

Turn: http: //www.2cto.com/database/201109/106249.html

 

oracle be used to comment on the command table or field descriptions, the following syntax:
the COMMENT the ON
  {TABLE [Schema.]
    {Table | View}
  | the COLUMN [Schema.]
    {Table | View | materialized_view} column...
  | OPERATOR [Schema .] operator
  | INDEXTYPE [Schema] the indextype.
  | MATERIALIZED the VIEW materialized_view
  }
the iS 'text';

used as follows:
1. Description table
Comment table table_name iS oN 'comments_on_tab_information';

2. Description of the columns in the table
comment on column table iS .column_name 'comments_on_col_information';

3. Display description
the SQL> SELECT * WHERE TABLE_NAME = USER_TAB_COMMENTS from 'the EMPLOYEES';

TABLE_NAME TABLE_TYPE the 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                     
FIRST_NAME the EMPLOYEES                     
the EMPLOYEES LAST_NAME                      
the EMPLOYEES TITLE                          
the EMPLOYEES SALARY staff salaries

SQL> from the SELECT * Comments USER_COL_COMMENTS the WHERE IS not null;

TABLE_NAME COLUMN_NAME COMMENTS
------------------------- ----- ------------------------------ -------------
the EMPLOYEES SALARY staff salaries

5. we can also see the table and column level from the following description of these views:
ALL_COL_COMMENTS
USER_COL_COMMENTS
ALL_TAB_COMMENTS
USER_TAB_COMMENTS 

6. delete table-level description, which is set to be empty
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
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