ORACLE SQL 高级子查询

第十八章:高级子查询
内外交互式子查询:
select ename,sal
from emp e
where sal>(select avg(sal) from emp where deptno=e.deptno);

存在否:exists
select ename
from emp
where empno in (select distinct mgr from emp);

select ename
from emp e
where exists
(select 'x' from emp where mgr=e.empno);

select ename
from emp e
where not exists
(select 'x' from emp where mgr=e.empno);

关联更新:
drop table e purge;
create table e as select * from emp;
alter table e add (location varchar2(13));
将正确的城市的信息添加到location列!
update e set location=
(select loc from dept where deptno=e.deptno);

关联删除:
从e表中删除销售部门的员工！
delete e where deptno=
(select deptno from dept where deptno='SALES');

with子句:
哪个部门总工资大于所有部门的平均工资!
SELECT *
FROM
(SELECT d.dname, SUM(e.sal) AS dept_total
FROM emp e, dept d
WHERE e.deptno = d.deptno
GROUP BY d.dname)
where dept_total>
(SELECT SUM(dept_total)/COUNT(*) AS dept_avg
FROM (SELECT d.dname, SUM(e.sal) AS dept_total
FROM emp e, dept d
WHERE e.deptno = d.deptno
GROUP BY d.dname));

WITH
dept_costs AS (
SELECT d.dname, SUM(e.sal) AS dept_total
FROM emp e, dept d
WHERE e.deptno = d.deptno
GROUP BY d.dname),
avg_cost AS (
SELECT SUM(dept_total)/COUNT(*) AS dept_avg
FROM dept_costs)
SELECT *
FROM dept_costs
WHERE dept_total >
(SELECT dept_avg
FROM avg_cost)
ORDER BY dname;

with子句中的视图叫做询问块，询问块的复杂查询在
with子句中只运行一次，运行成功后会将询问块的结果集
保存到temp表空间，以后再次调用询问块时会在后台转换
为对结果集的直接访问

使连续重复的值只显示一次:
with test as (
select 1 cd,'a' name from dual union all
select 1 cd,'b' name from dual union all
select 1 cd,'c' name from dual union all
select 1 cd,'d' name from dual union all
select 1 cd,'e' name from dual union all
select 2 cd,'q' name from dual union all
select 2 cd,'w' name from dual union all
select 2 cd,'e' name from dual)
select
(case when
(row_number() over(partition by cd order by name))=1
then cd else null end) as cd,name from test;