函数regexp_substr()&wm_concat()

1.需求：一个字段里存放了a,b,c,d,e这5个，但最终显示的时候我要计算他们出现的次数
regexp_substr函数，将一列转换成多行.
REGEXP_SUBSTR函数格式如下：
function REGEXP_SUBSTR(String, pattern, position, occurrence, modifier)
__srcstr     ：需要进行正则处理的字符串
__pattern    ：进行匹配的正则表达式
__position   ：起始位置，从第几个字符开始正则表达式匹配（默认为1）
__occurrence ：标识第几个匹配组，默认为1
__modifier   ：模式（'i'不区分大小写进行检索；'c'区分大小写进行检索。默认为'c'。）

select regexp_substr('SMITH,ALLEN,WARD,JONES,MARTIN,BLAKE,CLARK,SCOTT,KING,TURNER,ADAMS,JAMES,FORD,MIL','[^,]+',1,level)  
         as cl1 
  from dual 
  connect by 
         level<=length('SMITH,ALLEN,WARD,JONES,MARTIN,BLAKE,CLARK,SCOTT,KING,TURNER,ADAMS,JAMES,FORD,MIL')-  
        length(replace('SMITH,ALLEN,WARD,JONES,MARTIN,BLAKE,CLARK,SCOTT,KING,TURNER,ADAMS,JAMES,FORD,MIL',',',''))+1;

2.将分组内子元素统计到一个字段中，串联子项
--把列值以","号分隔起来,并显示成一行

select * from scott.emp;

--串联子项
select tt.deptno,tt.allchild
from(select d.deptno,WMSYS.WM_CONCAT(D.ENAME) OVER (partition by d.deptno order by d.ENAME) as allchild,
row_number() over(partition by d.deptno order by d.ENAME) as rn,
count(*) over(partition by d.deptno) as cn
from scott.emp d) tt
where tt.rn=tt.cn;

用replace()即可替换为其他分割：

select tt.deptno,tt.allchild
from(select d.deptno,replace(WMSYS.WM_CONCAT(D.ENAME)OVER (partition by d.deptno order by d.ENAME),',','|')  as allchild,
row_number() over(partition by d.deptno order by d.ENAME) as rn,
count(*) over(partition by d.deptno) as cn
from scott.emp d) tt
where tt.rn=tt.cn;

扩展用法:
案例:我要写一个视图,类似"create or replace view as select 字段1,...字段50 from tablename" ,基表有50多个字段,要是靠手工写太麻烦了,有没有什么简便的方法? 应用wm_concat来让这个需求变简单。将字段名称存入
user_tab_columns 表中，然后执行如下即可：

select 'create or replace view as select '|| wm_concat(column_name) || ' from dept'from user_tab_columns where table_name='DEPT';