Oracle數據庫9i、10g裏列轉行各種方法比較

把字符串 '1,2,3,4,5'，以逗號分隔，輸出爲行，也就是

1
2
3
4
5

隨手寫了一個, Oracle 10G 以上
有表如下

SQL> select * from t;
        ID NAME
---------- ----------------
         1 0,1,5,2,8,10
         2 9,7,8
         3 你好,他好,大家好

with vmaxnum as (
     select  rownum ele
     from    dual
     connect by rownum<=(select max(length(name)-length(replace(name,',',null)))+1 from t))
select id,
       decode(pos,0,substr(name,lagpos+1),substr(name,lagpos+1,pos-lagpos-1)) name
from   (select id,name,ele,pos,nvl(lag(pos) over (partition by id order by ele),0) lagpos
        from   (select id,name,ele,
                       instr(name,',',1,ele) pos
                from   (select /*+ all_rows no_merge(v2) use_merge(t,v2) */
                               t.id,t.name,v2.ele
                        from   t,vmaxnum v2
                        where  ele<=length(name)-length(replace(name,',',null))+1)))
order  by id,ele;

        ID NAME
---------- --------
         1 0
         1 1
         1 5
         1 2
         1 8
         1 10
         2 9
         2 7
         2 8
         3 你好
         3 他好
         3 大家好

12 rows selected.

然後客戶那反饋說數據庫說9i的，報錯說connect by不能使用子查詢。
於是再稍做修改，在確定逗號不會超過99個的前提下，直接寫個常量100，可以支持9i
 

with   vmaxnum as (
       select  rownum ele
       from    dual
       connect by rownum<=100)
select id,
       decode(pos,0,substr(name,lagpos+1),substr(name,lagpos+1,pos-lagpos-1)) name
from   (select id,name,ele,pos,nvl(lag(pos) over (partition by id order by ele),0) lagpos
        from   (select id,name,ele,
                       instr(name,',',1,ele) pos
                from   (select /*+ all_rows no_merge(v2) use_merge(t,v2) */
                               t.*,v2.ele
                        from   t,vmaxnum v2
                        where  ele<=length(name)-length(replace(name,',',null))+1)))
order  by id,ele;

後來想想這樣也不好，於是建議建立一張特定的IOT表，保存10000個數字，一般夠用了。
建立一張IOT,共10000行，同樣支持9i

create table tnumber(ele,constraint pk_tnumber primary key(ele)) organization index as
select rownum id from dual connect by rownum<=10000;

然後不再需要常量100，即保證準確，又保證較好的性能

create table t500 nologging as
with   vmaxnum as (
       select  ele
       from    tnumber
       where   ele<=(select max(length(name)-length(replace(name,',',null)))+1 from t))
select id,
       decode(pos,0,substr(name,lagpos+1),substr(name,lagpos+1,pos-lagpos-1)) name
from   (select id,name,ele,pos,nvl(lag(pos) over (partition by id order by ele),0) lagpos
        from   (select id,name,ele,
                       instr(name,',',1,ele) pos
                from   (select /*+ all_rows no_merge(v2) use_merge(t,v2) */
                               t.id,t.name,v2.ele
                        from   t,vmaxnum v2
                        where  ele<=length(name)-length(replace(name,',',null))+1)))
order  by id,ele;

        ID NAME
---------- --------
         1 0
         1 1
         1 5
         1 2
         1 8
         1 10
         2 9
         2 7
         2 8
         3 你好
         3 他好
         3 大家好

12 rows selected.

下面測試一下性能：轉換50萬行試試，測試環境：DELL D630 用了4年半的舊筆記本，磁盤都是碎片
SQL> insert into t select rownum+4,'1,2,3,4' from dual connect by rownum<=500000;

500000 rows created.

SQL> commit;

Commit complete.

set timi on
SQL> set timi on
SQL> create table t500 nologging as
     with   vmaxnum as (
            select  ele
            from    tnumber
            where   ele<=(select max(length(name)-length(replace(name,',',null)))+1 from t))
     select id,
            decode(pos,0,substr(name,lagpos+1),substr(name,lagpos+1,pos-lagpos-1)) name
     from   (select id,name,ele,pos,nvl(lag(pos) over (partition by id order by ele),0) lagpos
             from   (select id,name,ele,
                            instr(name,',',1,ele) pos
                     from   (select /*+ all_rows no_merge(v2) use_merge(t,v2) */
                                    t.id,t.name,v2.ele
                             from   t,vmaxnum v2
                             where  ele<=length(name)-length(replace(name,',',null))+1)))
     order  by id,ele;

Table created.

Elapsed: 00:00:16.48

耗時16秒，含建表的寫盤時間

SQL> select count(*) from t500;

  COUNT(*)
----------
   2000012

當然還有別的寫法，比如簡單的可以這樣

select  id,substr(name,instr(name,',',1,rownum)+1,instr(name,',',1,rownum+1)-instr(name,',',1,rownum)-1) name
from    (select  id,','||name||',' name from t)
connect by rownum<length(translate(name,','||name,','));

或者10G以上用正則表達式也可以實現
這個方法可能不是最高效的，但也還可以。
這個SQL的性能和字符串含有的逗號的個數有關，逗號越多，也就是分隔項越多，性能越差。