Reg: WMSYS.WM_CONCAT or LISTAGG

Question

All,
I am trying to select the column names which start with 'E' in my sql query projection area. For this i have used USER_TAB_COLS with WMSYS.WM_CONCAT / LISTAGG too.
I got the column names, with ',' delimeter, but not able inject this to the sql query. Please help me.
SQL> SELECT WMSYS.WM_CONCAT(column_name)
  2    FROM user_tab_cols
  3    WHERE table_name='EMP'
  4    AND column_name LIKE 'E%';
WMSYS.WM_CONCAT(COLUMN_NAME)
EMPNO,ENAME
SQL> SELECT
  2    (SELECT WMSYS.WM_CONCAT(column_name)
  3    FROM user_tab_cols
  4    WHERE table_name='EMP'
  5    AND COLUMN_NAME LIKE 'E%'
  6    ) a
  7  FROM EMP where rownum<=4;
A
EMPNO,ENAME
ERROR:
ORA-22922: nonexistent LOB value
---LISTAGG
SQL> select (select ListAgg(column_name,',')
  2         WITHIN GROUP(ORDER BY COLUMN_NAME DESC) AS CONCATV
  3  from user_tab_cols
  4    WHERE TABLE_NAME='EMP'
  5    AND COLUMN_NAME LIKE 'E%') a FROM EMP WHERE ROWNUM<=3;
A
ENAME,EMPNO
ENAME,EMPNO
ENAME,EMPNO
Thanks,

Answer 1

The SQL Projection must be defined and known to the SQL Engine during compilation time. So you cant pick a column name dynamically and expect it to return its values.
So you have two options to achieve what do you want.
1. Dynamic SQL
2. XML
Example for Dynamic SQL has already been shown to you. Below is a example for obtaining to the output as XML.
SQL> select * from v$version where rownum = 1;
BANNER
Oracle Database 10g Enterprise Edition Release 10.2.0.5.0 - 64bi
SQL> select dbms_xmlgen.getxmltype
  2         (
  3             'select ' || ltrim(sys_connect_by_path(column_name, ','), ',') ||
  4             '  from ' || table_name
  5         ) xml_output
  6    from (
  7            select table_name
  8                 , column_name
  9                 , row_number() over(partition by table_name order by column_name) rno
10              from user_tab_columns
11             where table_name = 'EMP'
12               and column_name like 'E%'
13         )
14   where connect_by_isleaf = 1
15   start
16    with rno = 1
17  connect by rno = prior rno + 1
18     and table_name = prior table_name;
XML_OUTPUT
<ROWSET>
<ROW>
  <EMPNO>7369</EMPNO>
  <ENAME>SMITH</ENAME>
</ROW>
<ROW>
  <EMPNO>7499</EMPNO>
  <ENAME>ALLEN</ENAME>
</ROW>
<ROW>
  <EMPNO>7521</EMPNO>
  <ENAME>WARD</ENAME>
</ROW>
<ROW>
  <EMPNO>7566</EMPNO>
  <ENAME>JONES</ENAME>
</ROW>
<ROW>
  <EMPNO>7654</EMPNO>
  <ENAME>MARTIN</ENAME>
</ROW>
<ROW>
  <EMPNO>7698</EMPNO>
  <ENAME>BLAKE</ENAME>
</ROW>
<ROW>
  <EMPNO>7782</EMPNO>
  <ENAME>CLARK</ENAME>
</ROW>
<ROW>
  <EMPNO>7788</EMPNO>
  <ENAME>SCOTT</ENAME>
</ROW>
<ROW>
  <EMPNO>7839</EMPNO>
  <ENAME>KING</ENAME>
</ROW>
<ROW>
  <EMPNO>7844</EMPNO>
  <ENAME>TURNER</ENAME>
</ROW>
<ROW>
  <EMPNO>7876</EMPNO>
  <ENAME>ADAMS</ENAME>
</ROW>
</ROWSET>
SQL>
My version of oracle does not support LISTAGG. So i have used SYS_CONNECT_BY_PATH. But you can use LISTAGG to achieve the same.