oracle中用comment on命令給表或字段加以說明，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.對表的說明
comment on table table_name is 'comments_on_tab_information';

2.對表中列的說明
comment on column table.column_name is 'comments_on_col_information';

3.查看表的說明
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.查看表中列的說明
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視圖中查看表級和列級說明：
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 ? ?

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