Oracle特殊查詢 行列倒轉 分頁

--查詢工資最高的前三名 (分頁的感覺)
select * from
(select * from emp order by sal desc) t
where rownum <=3
--查詢工資最高的4到6名 (分頁-->排序 行號 選擇三步)
select *
from (select t.*,rownum rn from (select * from emp order by sal desc) t) m where m.rn >= 4 and m.rn<=6

select *
from (select t.*,rownum rn from (select * from emp order by sal desc) t) m where m.rn between 4 and 6

select *
from (select e.*,row_number() over(order by e.sal desc) rn from emp e) t
where t.rn between 4 and 6
--查詢每年入職的員工個數
select count(*), to_char(hiredate,'yyyy') from emp group by to_char(hiredate,'yyyy')
--把上面查詢的表行列倒轉
select count(*), to_char(hiredate,'yyyy') from emp group by to_char(hiredate,'yyyy')

select
sum(num) "Total",
avg(decode(hireyear,'1980',num)) "1980",
sum(decode(hireyear,'1981',num)) "1981",
max(decode(hireyear,'1982',num)) "1982",
min(decode(hireyear,'1987',num)) "1987"
from
(select count(*) num, to_char(hiredate,'yyyy') hireyear from emp group by to_char(hiredate,'yyyy')) t
--交集 兩個集合共同的元素
select * from emp where deptno=20
intersect
select * from emp where sal>2000
--並集 兩個集合所有的元素
--不去重
select * from emp where deptno=20
union all
select * from emp where sal>2000
--去重
select * from emp where deptno=20
union
select * from emp where sal>2000
--差集 A有B沒有的元素
select * from emp where deptno=20
Minus
select * from emp where sal>2000