Java38: 数据库二(Oracle)

group by 分组

--每个工作的人数
select count(job) from emp group by job;
--有多少工作
select count(coutn(job)) from emp group by job;
--每个工作的最高工资
select max(sal) from emp group by job;
--每个工作的最低工资
select min(sal) from emp group by job;
--每个工作的平均工资
select avg(sal) from emp group by job;
--所有工作中最大的平均工资
select max(avg(sal)) from emp group by job;

使用having 根据条件过滤组

select job,count(job) from emp group by job having count(job)>=3;

where 用在group by 之前

select job,count(job) from emp where sal>1000 group by job having count(job)>=3;

decode 函数

decode(value1 if1 then1 if2 then2 ... else)

decode(条件,值1,返回值1,值2,返回值2,...值n,返回值n,缺省值) 如果 value1=if1 返回then1 否则 如果 value1=if2 返回 then2
默认的返回值是else

select decode(sal,1600,'一千六',5000,'五千','其他') from emp;

--按照job的内容分类统计其中部门ID是30的人数
select job,count(decode(DEPTNO,30,1,null)) deptno from emp group by(job);
--按照部门分类 统计每个部门中工作是CLERK的人数
select deptno,count(decode(job,'CLERK',1,null)) counts from emp group by(deptno);
--按照部门分类 计算每个每部中工作是CLERK的平均工资
select deptno,avg(decode(job,'CLERK',sal,null)) CLERK_AVG_SAL from emp group by(deptno);

insert into emp values(8000,'lmdtx','CLERK',8080,to_date('1980/09/09','yyyy/mm/dd'),30000,null,null);
insert into emp values(8880,'dsy','CLERK',8980,to_date('1982/02/02','yyyy/mm/dd'),3000,null,null);

表的关联
外键 在一个表中 存着对应另外一张表的主键
连接表 1·笛卡尔积 emp 表中 deptno 为空的查询不到

--emp表中的所有的行* dept中的所有的行
select * from emp,dept;
--需要写过滤条件 过滤掉不需要的
select * from emp,dept where emp.deptno=dept.deptno;

select * from emp e.dept d where e.deptno=d.deptno;

2·内连接 这个结果和笛卡尔积 用条件后的结果一样
inner join 和 join 一样（省略 inner） 默认就是 内连接

join...on...
from emp e inner join dept d on e.deptno=d.deptno

两个表的一个外键的名字 和 另一张表的主键的名字一样

from emp inner join dept using(deptno)

select * from emp e inner join dept d on e.deptno=d.deptno;

select * from emp inner join dept using(deptno);

3·外连接
1·左外连(左边表的数据必须是全都有的，没有的地方补null)

from emp left outer join dept using(deptno)

select * from emp left outer join dept on emp.deptno=dept.deptno;
select ename,nvl(dname,'还没有部门') from emp2 left outer join dept2 using(deptno);

2·右外连

select * from emp right outer join dept using(deptno);

--查询所有部门的名称和部门人数按人数的降序排
select dname,count(empno) num
from emp2 right join dept2 Using(deptno)
group by dname
order by num desc;

3·全外连（全都有）

emp full outer join dept

select * from emp full outer join dept using(deptno);
--查询所有部门的名称以及部门的人数 没有部门的人 化为 未分配
select nvl(dname,'未分配'),count(empno)
from emp2 full outer join dept2 using(deptno)
group by dname
order by dname;

select nvl(dname,'未分配部门') 部门名字,decode(count(empno),0,'未分配人手',count(empno)) 人数
from emp2 full outer join dept2 using(deptno)
group by dname
order by dname;

4·自连接

select e1.ename,e2.ename from emp2 e1 inner join emp2 e2 on e1.mgr=e2.empno;

select e1.ename, e2.ename from emp2 e1 left join emp2 e2 on e1.mgr = e2.empno;
select nvl(e1.ename,'没有手下')  , e2.ename MGR from emp2 e1 right join emp2 e2 on e1.mgr=e2.empno;

select e2.ename,decode(count(e1.ename),0,'没有手下',count(e1.ename))from emp2 e1 right join emp2 e2 on e1.mgr=e2.empno group by e2.ename

子查询

用在where 子句中，单行单列
select e1.ename from emp2 e1 inner join emp2 e2 on e1.mgr=e2.empno where e2.ename='KING';

select distinct dname from emp2 e1 join dept2 d2 ON e1.deptno= d2.deptno where e1.ename like '%S%';

select dname from dept2 where deptno in(select deptno from emp2 where ename like '%S%');

ANY() 任意一个ALL() 所有

select ename,job,sal from emp2 where sal>all(select DISTINCT sal from emp2 where job='MANAGER') and job<>'MANAGER';

select ename job,sal from emp2 where sal>any(select distinct sal from emp2 where job='CLERK') and job<>'CLERK';

exists 用于判断后面的子查询是否有行 有true 没有false

select ename from emp2 e1 where EXISTS(select 1 from emp2 e2 where e2.mgr= e1.empno);

select ename from emp2 e1 where not exists(select 1 from emp2 e2 where e2.mgr=e1.empno);

3 in (1,2,3) true3 in (1,2,null) false5 not in (1,2,3) true5 not in (1,2,null) false5 in (1,2,null) false5 not in (null) false5in (null) false
not in 的子查询中不能出现null值

where 中的多列 查询

select e.ename,e.sal, d.deptno,d.dname from emp2 e inner join dept2 d on e.deptno= d.deptno where (e.deptno,e.sal) in (select deptno,min(sal) from emp2 where deptno is not null group by deptno);

select * from emp2 where (deptno,sal)in (select deptno,min(sal) from emp2 where deptno is not null group by deptno);

select deptno,min(sal) from emp2 where deptno is not null group by deptno;

用在having

from emp where deptno is not null group by deptno having avg(sal)<(select max(avg(sal)) from emp group by deptno)

select max(avg(sal)) from emp2 group by deptno;

select deptno,round(avg(sal)) from emp2 where deptno is not null group by deptno HAVING avg(sal)<(select max(avg(sal)) from emp2 group by deptno);

select dname,deptno from emp2 join dept2 using(deptno) where deptno is not null group by deptno,dname having avg(sal)=(select max(avg(sal)) from emp2 group by deptno);

分页
伪列

select empno,ename,rownum from emp2 order by ename;

select * from emp2 where rownum<=10;

select * from (select empno,ename, rownum n from emp2 where rownum<=10) where n>5 and n<=10

select * from (select empno,ename, rownum n from emp2 ) where n>5 and n<=10

select * from (select empno,ename, rownum n from emp2 where rownum<=10) where n>5

select * from (select empno,ename,sal, rownum n from (select * from emp2 order by sal) where rownum <=10) where n>5;

内联视图(Inner View)
就是把select后的结果当作一张表

select ename 名字,job 职位,emp_num 手下人数 from emp2 join(select deptno d,count(empno) emp_num from emp2 inner join dept2 using(deptno) group by deptno) on deptno=d where job='MANAGER';

select ename 名字，job 职位,dname 部门,deptno 部门编号,emp_num 手下人数 from emp2 inner join(select deptno d,count(empno) emp_num,dname from emp2 inner join dept2 using(deptno) group by deptno, dname) on deptno=d where job='MANAGER'

group by 对多个列分组

而这两个列都是对部门的唯一标识，不会影响分组