oracle case when 字符串转行

UPDATE tablename SET PARENTID=(
       case
       WHEN LENGTH(column1)/2=1 then '0'
       when LENGTH(column2)/2=2 then substr(column1,1,2)
       when LENGTH(column3)/2=3 then substr(column2,1,4)
       when LENGTH(column4)/2=4 then substr(column3,1,6)
       end
)


select
       userid,
       substr(userid,
              instr(',' || userid || ',', ',', 1, n),
              instr(',' || userid || ',', ',', 1, n + 1) -
              instr(',' || userid || ',', ',', 1, n) - 1) aa
  from xt_jbpm_dbsxb, (select level n from dual connect by level <= 10) x

where substr(userid,
              instr(',' || userid || ',', ',', 1, n),
              instr(',' || userid || ',', ',', 1, n + 1) -
              instr(',' || userid || ',', ',', 1, n) - 1) is not null
and id=77823
order by n