oracle comment on的用法

oracle中用comment on命令給表或欄位加以說明，語法如下：

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.對錶的說明

comment on table table_name is 'comments_on_tab_information';

2.對錶中列的說明

comment on column table.column_name is 'comments_on_col_information';

3.檢視錶的說明

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

TABLE_NAME                     TABLE_TYPE  COMMENTS
------------------------------ ----------- ----------
EMPLOYEES                      TABLE       員工表

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

TABLE_NAME                     TABLE_TYPE  COMMENTS
------------------------------ ----------- --------------------------
EMPLOYEES                      TABLE       員工表

4.檢視錶中列的說明

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                      
EMPLOYEES                      TITLE                          
EMPLOYEES                      SALARY                         員工薪水

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

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

5.我們也可以從下面這些檢視中檢視錶級和列級說明：

ALL_COL_COMMENTS
USER_COL_COMMENTS
ALL_TAB_COMMENTS
USER_TAB_COMMENTS 

6.刪除表級說明，也就是將其置為空

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.刪除列級說明，也是將其置為空

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