对某一用户解锁：
现已管理员system身份登录，
alter user hr account unlock;
重设密码为hr：
alter user hr identified by hr
连接hr
connect hr/hr
连接查询：
select t1.first_name,t1.last_name,t1.job_id,t1.salary,t2.department_name
from employees t1, departments t2 where
t2.department_name='Sales';
使用子查询查询某一部门的员工信息
select t1.first_name ,t1.last_name,t1.job,t1.salary
from emplyees t1;
统计某一部门的雇员的最高和最低工薪
select t2.department_name,max(t1.salary),min(t1.salary)
from employees t1,departments t2
where t1.department_id = t2.department_id
group by t2.department_name;
向employees表中添加一组数据
insert into employees(employee_id,last_name,email,hire_date,job_id,salary)
values(300,'LILI','lili@',sysdate,'SH_CLERK',1250);
删除上述记录
delete employees
where employee_id=300;
为部门编号为50的员工上调工薪20%
update employees
set salary=salary*1.2
where department_id=50;
创建表的副本并添加数据
create table employees_copy
as select * from employees;
使用truncate语句清除表中的数据
truncate table employees_copy
