Oracle Sql優化之Rownum的使用

1.rownum:rownum是一個偽列，需要在數據取出來后，rownum才會有值，因此在分頁查找時，需要進行嵌套查詢。

select sal,ename from
 (select rownum as rn,sal,ename from    
   (select sal,ename from emp where sal is not null order by sal) x
 where rownum<10)
where rn>6

采用分析函數也是可以實現一次嵌套

select rn,ename,sal from   
   (select rownum() over(order by sal) as rn,sal,ename where sal is not null ) x
where rn between 6 and 10

但是由於分析函數的影響，有些索引可能失效，建議大家采用第一種寫法。

隔行返回數據,對偽列求余即可,MOD(rn,X)

2.Merge:高效的表更新處理

Merge inot test a
using
 (select rowid as rid,nbr*100+rownum() over(partition by nbr order by rowid) as nnbr from test) b
on(a.rowid=b.rid)
when matched then
update set a.nbr = b.nnbr;

大家猜猜 test表掃描了幾次?????

3.將表中某些列，排列組合去重

step1:列轉行

select * from test unpivot(b2 for b3 in(t1,t2,t3)

unpivot行列轉置非常牛逼的一個方法

step2:按照值排序並合並

with x1 as 
  (select * from test unpivot(b2 for b3 in(t1,t2,t3)),
select id,listagg(b2,',') within group (order by b2) as b
from x1
group by id;

listagg實現分組后，值排序后，值按逗號拼接

step3:去除重復

with x1 as 
  (select * from test unpivot(b2 for b3 in(t1,t2,t3)),
x2 as
( select id,listagg(b2,',') within group (order by b2) as b 
  from x1
  group by id)
select id,b,row_number() over(partition by b order by id) as sn from x2