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oracle中用comment on命令给表或字段加以说明，语法如下：

用法如下：

1.对表的说明

2.对表中列的说明

3.查看表的说明

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 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：

6.删除表级说明，也就是将其置为空

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> 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