java开发常见笔试题(数据库篇)

一.根据下列员工信息表和部门信息表完成以下SQL查询
(表1 emp_xiangyoulu 员工信息表)
(表2 dept_xiangyoulu)
1.查询所有员工信息
Select * from emp_xianyoulu;
2.查看所有部门信息
Select * from dept_xiangyoulu;
3:查看公司工资排名的第2-5名(分页查询rownum )
select * from(
select rownum rn,ename,sal from emp_xiangyoulu order by sal desc) where rn between 2 and 5;--->错! 伪列rownum和排序order by 同时出现时,先生成伪列号,再按条件排序--->导致rn对应的序号在排序之前就定了,查出的结果和预期不符;;
SELECT ROWNUM rn,e.* FROM (
SELECT * FROM emp_xiangyoulu ORDER BY sal desc) e
WHERE rn BETWEEN 2 AND 5;--->错! "RN": 标识符无效,伪列号不能直接当成一般属性直接用;
select rownum,e.* from(
select * from emp_xiangyoulu order by sal desc)e
where rownum between 2 and 5;--->错! 不报错,但查询结果为0条,原因同上;
select * from(
select rownum,e.* from(
select * from emp_xiangyoulu order by sal desc)e)
where rownum between 2 and 5;--->错! 不报错,但查询结果为0条,rownum 没取别名,直接用有影响;
select * from(
select rownum rn,e.* from(
select * from emp_xiangyoulu order by sal desc)e)
where rn between 2 and 5;--->结果正确!!!
:查看奖金为NULL的员工
select * from emp_xiangyoulu where comm is null;(不能用=,=只能判断是否为" "空字符串)
4.数据库中出现两条一样的数据,如何只留一条
DISTINCT关键字(distinct)去重;
在SELECT子句中使用，用来对指定的字段值去除重复行。

查看公司有哪些职位
SELECT DISTINCT job FROM emp;
多字段去重时，不保证单一的某个字段的值没有重复，而去重原则是这些字段值的
组合没有重复行。

SELECT DISTINCT job,deptno FROM emp
删除除了ID之外,其他都一样的冗余数据(重复数据只保留一条);
delete student where (code,name) in(
select code,name from student group by code,name having
count(name)>1)
and id not in(
select min(id) from student group by code,name having
count(name)>1);
delete student where id not in(
select min(id) from student group by code,name);
select * from student;
5.查询最低薪水高于30号部门最低工资的部门信息
SELECT deptno, MIN(sal) min_sal FROM emp
GROUP BY deptno
HAVING min_sal >(SELECT MIN(sal)
FROM emp WHERE deptno = 30);
询部门平均薪资大于2000的所有员工信息;
select * from emp where deptno in (
select deptno from emp group by deptno
hiving AVG(sal)>2000
);
9.查询每个部门每种职位的员工人数;
Select count(job),deptno,job
from emp_xiangyoulu
group by rollup(deptno,job);
10:查看从2008-08-08号到今天为止一共经历了多少天
SELECT SYSDATE-TO_DATE('2008-08-08','YYYY-MM-DD')
FROM DUAL;
--向上取整；；
--SELECT CEIL(SYSDATE-TO_DATE('2008-08-08','YYYY-MM-DD'))
FROM DUAL;
11:将每名员工入职时间以例如:(1981年12月3日的形式显示)
select ename,to_char(hiredate, 'yyyy"年"MM"月"dd"日"')
from emp_xiangyoulu;
12:查看SMITH的上司在哪个城市工作(三表关联查询)
Select ,,,,
from emp_xiangyoulu e,emp_xiangyoulu m,dept_xiangyoulu d
Where = and = and ='smith';
13:查看平均工资高于2000的那些部门名字以及所在城市
select ,avg,, from emp_xiangyoulu e,dept_xiangyoulu d
where = group by ,, having avg>2000;
14.在NEW YORK工作的员工有多少人
select count, from emp_xiangyoulu e,dept_xiangyoulu d
where = and ='NEW YORK' group by ;
15:查看低于自己所在部门平均工资的员工(将查询结果作为表来关联查
询 )
select ,, from emp_xiangyoulu e,(
select avg(sal) a_sal,deptno from emp_xiangyoulu group by deptno) d where = and < ;。