在使用Listagg时候，报ORA-01489错误。原来是字符太长超过4000。后找得解决方案如下：
原文在此处：
http://stackoverflow.com/questions/11541383/ordering-by-list-of-strings-in-oracle-sql-without-listagg

都是IT人员，直接上代码：
SQL> CREATE TYPE tab_varchar2 AS TABLE OF VARCHAR2(4000);
  2  /

Type created.

SQL> CREATE OR REPLACE FUNCTION concat_array(p tab_varchar2) RETURN CLOB IS
  2     l_result CLOB;
  3  BEGIN
  4     FOR cc IN (SELECT column_value FROM TABLE(p) ORDER BY column_value) LOOP
  5        l_result := l_result ||' '|| cc.column_value;
  6     END LOOP;
  7     return l_result;
  8  END;
  9  /

Function created.

SQL> SELECT item,
  2         concat_array(CAST (collect(attribute) AS tab_varchar2)) attributes
  3    FROM data
  4   GROUP BY item;

ITEM ATTRIBUTES
1    a b c
2    a c
3    a b