#yyds干货盘点#oracle常见面试题

数据库对象：表(table) 视图(view) 序列(sequence) 索引(index) 同义词(synonym)

1.视图: 存储起来的 select 语句

create view emp_vw
as
select employee_id, last_name, salary
from employees
where department_id = 90;

select * from emp_vw;

可以对简单视图进行 DML 操作

update emp_vw
set last_name = 'HelloKitty'
where employee_id = 100;

select * from employees
where employee_id = 100;

1). 复杂视图

create view emp_vw2
as
select department_id, avg(salary) avg_sal
from employees
group by department_id;

select * from emp_vw2;

复杂视图不能进行 DML 操作

update emp_vw2
set avg_sal = 10000
where department_id = 100;

2.序列：用于生成一组有规律的数值。（通常用于为主键设置值）

create sequence emp_seq1
start with 1
increment by 1
maxvalue 10000
minvalue 1
cycle
nocache;

select emp_seq1.currval from dual;

select emp_seq1.nextval from dual;

问题：裂缝，原因：

• 当多个表共用同一个序列时。
• rollback
• 发生异常

create table emp1(
     id number(10),
     name varchar2(30)
);

insert into emp1
values(emp_seq1.nextval, '张三');

select * from emp1;

3.索引：提高查询效率

自动创建：Oracle 会为具有唯一约束(唯一约束，主键约束)的列，自动创建索引

create table emp2(
       id number(10) primary key,
       name varchar2(30)
)

手动创建

create index emp_idx
on emp2(name);

create index emp_idx2
on emp2(id, name);

4.同义词

create synonym d1 for departments;

select * from d1;

5.表

DDL ：数据定义语言 create table .../ drop table ... / rename ... to..../ truncate table.../alter table ...

DML : 数据操纵语言

insert into ... values ...
update ... set ... where ...
delete from ... where ...

【重要】

select ... 组函数(MIN()/MAX()/SUM()/AVG()/COUNT())
from ...join ... on ... 左外连接：left join ... on ... 右外连接: right join ... on ...
where ...
group by ... (oracle,SQL server中出现在select 子句后的非分组函数，必须出现在 group by子句后)
having ... 用于过滤 组函数
order by ... asc 升序， desc 降序
limit (0,4) 限制N条数据 如: topN数据
- union 并集
- union all(有重复)
- intersect 交集
- minus 相减
DCL : 数据控制语言  commit : 提交 / rollback : 回滚 / 授权grant...to...  /revoke

SQL 优化：能使用 EXISTS 就不要使用 IN

问题：查询公司管理者的employee_id,last_name,job_id,department_id信息

select employee_id, last_name, job_id, department_id
from employees
where employee_id in (
      select manager_id
      from employees
)

select employee_id, last_name, job_id, department_id
from employees e1
where exists (
      select 'x'
      from employees e2
      where e1.employee_id = e2.manager_id
)

问题：查询departments表中，不存在于employees表中的部门的department_id和department_name

select department_id, department_name
from departments d1
where not exists (
      select 'x'
      from employees e1
      where e1.department_id = d1.department_id
)

更改 108 员工的信息: 使其工资变为所在部门中的最高工资, job 变为公司中平均工资最低的 job

update employees e1
set salary = (
    select max(salary)
    from employees e2
    where e1.department_id = e2.department_id
), job_id = (
   select job_id
   from employees
   group by job_id
   having avg(salary) = (
         select min(avg(salary))
         from employees
         group by job_id
   )
)
where employee_id = 108;

删除 108 号员工所在部门中工资最低的那个员工.

delete from employees e1
where salary = (
      select min(salary)
      from employees
      where department_id = (
            select department_id
            from employees
            where employee_id = 108
      )
)

select * from employees where employee_id = 108;
select * from employees where department_id = 100
order by salary;

rollback;