How can I replace this listagg function to xmlagg

Question

I want to replace the below query using XMLAGG function and also eliminate the distinct logic.

SELECT
 listagg(distinct OD.PROD_NAME,';') within group(order by SC.CATEGORY_ID) as PRODUCT
 FROM 
 orders OD,
SERVICES SC

    

I tried to refer below query but couldn't understand

select
   deptno,
   rtrim (xmlagg (xmlelement (e, ename || ',')).extract ('//text()'), ',') enames
from
   emp
group by
   deptno
;

Answer 1

You tagged the question with "Oracle 11g" tag; it doesn't support LISTAGG (DISTINCT...) - distinct was introduced in 19c so - if you aren't on that version (or higher), you can't use it.

What to do, then? Use a subquery to fetch distinct values you need, and the join it (the subquery) to another table.

Here's an example based on Scott's sample schema (as I don't have your tables):

SQL> SELECT LISTAGG (e.job, ';') WITHIN GROUP (ORDER BY d.deptno, e.job) result
  2    FROM (SELECT DISTINCT deptno, job
  3            FROM emp) e
  4         JOIN dept d ON d.deptno = e.deptno;

RESULT
------------------------------------------------------------------------------------------------------------------------
CLERK;MANAGER;PRESIDENT;ANALYST;CLERK;MANAGER;CLERK;MANAGER;SALESMAN

SQL>
SQL>
SQL> SELECT RTRIM (
  2            XMLAGG (XMLELEMENT (e, e.job || ';') ORDER BY d.deptno, e.job).EXTRACT (
  3               '//text()'),
  4            ';') result
  5    FROM (SELECT DISTINCT deptno, job
  6            FROM emp) e
  7         JOIN dept d ON d.deptno = e.deptno;

RESULT
------------------------------------------------------------------------------------------------------------------------
CLERK;MANAGER;PRESIDENT;ANALYST;CLERK;MANAGER;CLERK;MANAGER;SALESMAN

SQL>

Or, one below another (for better visibility):

LISTAGG: CLERK;MANAGER;PRESIDENT;ANALYST;CLERK;MANAGER;CLERK;MANAGER;SALESMAN     
XMLAGG : CLERK;MANAGER;PRESIDENT;ANALYST;CLERK;MANAGER;CLERK;MANAGER;SALESMAN  

As you can see, both return the same result.

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Can I suggest a query for your case? Nope; as I said, I don't have your tables and I have no idea how orders and services are related to each other.