Oracle各种连接函数总结

1.前言

Oracle可用连接函数会介绍以下几个

1. Oracle列转行函数 Listagg()
2. strcat()
3. wmsys.wm_concat()

2.Oracle列转行函数 Listagg()

2.1最基础的用法：

LISTAGG(XXX,XXX) WITHIN GROUP( ORDER BY XXX)
用法就像聚合函数一样，通过Group by语句，把每个Group的一个字段，拼接起来。
其中LISTAGG函数第一个参数为要拼接的字段,第二个参数为用什么字符串进行连接
eg : listagg(city,’,’)
后面GROUP()中为对连接之后的行数据按什么字段进行排序
eg : order by city

执行SQL : 
    with temp as(  
    select 'China' nation ,'Guangzhou' city from dual union all  
    select 'China' nation ,'Shanghai' city from dual union all  
    select 'China' nation ,'Beijing' city from dual union all  
    select 'USA' nation ,'New York' city from dual union all  
    select 'USA' nation ,'Bostom' city from dual union all  
    select 'Japan' nation ,'Tokyo' city from dual   
    )  
     select nation,listagg(city,',') within GROUP (order by city)  as Cities
        from temp  
        group by nation

运行结果：

2.2同样是聚合函数，还有一个高级用法：

就是over(partition by XXX)
也就是说，在你不实用Group by语句时候，也可以使用LISTAGG函数：

with temp as(  
  select 500 population, 'China' nation ,'Guangzhou' city from dual union all  
  select 1500 population, 'China' nation ,'Shanghai' city from dual union all  
  select 500 population, 'China' nation ,'Beijing' city from dual union all  
  select 1000 population, 'USA' nation ,'New York' city from dual union all  
  select 500 population, 'USA' nation ,'Bostom' city from dual union all  
  select 500 population, 'Japan' nation ,'Tokyo' city from dual   
)  
select population,  
       nation,  
       city,  
       listagg(city,',') within GROUP (order by city) over (partition by nation) rank  
from temp

运行结果：

2.3总结

listagg()函数支持最低版本需要Oracle 11gR2,查询自己Oracle版本sql如下,

SELECT v.VERSION FROM v$instance v;

如果版本低于11g,查询会报错 [未找到要求的 FROM 关键字]

3.strcat()

执行SQL:
  with temp as(  
        select 'China' nation ,'Guangzhou' city from dual union all  
        select 'China' nation ,'Shanghai' city from dual union all  
        select 'China' nation ,'Beijing' city from dual union all  
        select 'USA' nation ,'New York' city from dual union all  
        select 'USA' nation ,'Bostom' city from dual union all  
        select 'Japan' nation ,'Tokyo' city from dual   
        )  
select nation,strcat(city) from temp  
group by nation

结果为:

注意点:如果执行报错,报错内容为 strcat标识符无效,则你的版本缺少这个函数,手动执行下面的strcat源码即可

ORACLE 字符串聚合函数 strCat

源码如下：
create or replace type strcat_type as object 
      (
        currentstr varchar2(4000), 
        currentseprator varchar2(8), 
        static function ODCIAggregateInitialize(sctx IN OUT strcat_type) return number,
        member function ODCIAggregateIterate(self IN OUT strcat_type,value IN VARCHAR2) return number,
        member function ODCIAggregateTerminate(self IN strcat_type,returnValue OUT VARCHAR2, flags IN number) return number,
        member function ODCIAggregateMerge(self IN OUT strcat_type,ctx2 IN strcat_type) return number
      );
/

  create or replace type body strcat_type is 
      static function ODCIAggregateInitialize(sctx IN OUT strcat_type) return number is 
      begin
        sctx := strcat_type('',',');
        return ODCIConst.Success;
      end;
      member function ODCIAggregateIterate(self IN OUT strcat_type, value IN VARCHAR2) return number is
      begin
        if self.currentstr is null then
           self.currentstr := value;
        else
          self.currentstr := self.currentstr ||currentseprator || value;
        end if;
        return ODCIConst.Success;
      end;
      member function ODCIAggregateTerminate(self IN strcat_type, returnValue OUT VARCHAR2, flags IN number) return number is
      begin
        returnValue := self.currentstr;
        return ODCIConst.Success;
      end;
      member function ODCIAggregateMerge(self IN OUT strcat_type, ctx2 IN strcat_type) return number is
      begin
        if ctx2.currentstr is null then
          self.currentstr := self.currentstr;
        elsif self.currentstr is null then 
          self.currentstr := ctx2.currentstr;
        else
          self.currentstr := self.currentstr || currentseprator || ctx2.currentstr;
        end if; 
        return ODCIConst.Success;
      end;
      end;
      
/
  CREATE OR REPLACE FUNCTION strcat (input VARCHAR2) RETURN VARCHAR2 PARALLEL_ENABLE AGGREGATE USING strcat_type;

/

4.wmsys.wm_concat()

注意:11gr2和12C上已经摒弃了wm_concat函数,所以要用连接函数,建议使用之前介绍的两种.如果之前老项目使用了这个函数,需要重建该函数或者在当前运行oracle版本中没有这个函数请看这 “WM_CONCAT”: 标识符无效

执行SQL:
 with temp as(  
        select 1 grp, 'a1' str from dual
        union
        select 1 grp, 'a2' str from dual
        union
        select 2 grp, 'b1' str from dual
        union
        select 2 grp, 'b2' str from dual
        union
        select 2 grp, 'b3' str from dual
        ) 
 select grp, wmsys.wm_concat(str)
  from temp
 group by grp

参考文章:

Oracle 列转行函数 Listagg()
strcat()源码转自:http://blog.itpub.net/3090/viewspace-625084/