oracle-comment on的用法

转载：https://blog.youkuaiyun.com/wangmuming/article/details/28878461
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
EMPLOYEES TITLE
EMPLOYEES SALARY 员工薪水

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

TABLE_NAME COLUMN_NAME COMMENTS

---

EMPLOYEES SALARY 员工薪水

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 ”;
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