WM_CONCAT with DISTINCT Clause - Compiled Package versus Stand-Alone Query Issue - oracle

Question

I was writing some program that uses the WM_CONCAT function. When I run this query:
SELECT WM_CONCAT(DISTINCT employee_id)
FROM employee
WHERE ROWNUM < 20;
It works fine. When I try to compile the relatively same query in a package function or procedure, it produces this error: PL/SQL: ORA-30482: DISTINCT option not allowed for this function
FUNCTION fetch_raw_data_by_range
RETURN VARCHAR2 IS
v_some_string VARCHAR2(32000);
BEGIN
SELECT WM_CONCAT(DISTINCT employee_id)
INTO v_some_string
FROM employee
WHERE ROWNUM < 20;
RETURN v_some_string;
END;
I realize WM_CONCAT is not officially supported, but can someone explain why it would work as a stand alone query with DISTINCT, but not compile in a package?

Answer 1

Problem is that WM_CONCAT is stored procedure written on pl/sql.
There is a open bug #9323679: PL/SQL CALLING A USER DEFINED AGGREGRATE FUNCTION WITH DISTINCT FAILS ORA-30482.
Workaround for problems like this is using dynamic sql.
So if you wrap your query in
EXECUTE IMMEDIATE '<your_query>';
Then it should work.
But as OldProgrammer has suggested already, you better avoid using this WM_CONCAT at all.

Answer 2

PL/SQL will not let you to use distinct in an aggregated function, and this issue shows that the SQL-engine and the PL/SQL-engine do not use the same parser.
One of the solutions to this problem is to use sub query as below,
SELECT WM_CONCAT(employee_id)
INTO v_some_string
FROM (select DISTINCT employee_id
FROM employee)
WHERE ROWNUM < 20;
Another solution is to use dynamic SQL as Nagh suggested,
FUNCTION fetch_raw_data_by_range
RETURN VARCHAR2 IS
v_some_string VARCHAR2(32000);
v_sql VARCHAR2(200);
BEGIN
v_sql :='SELECT WM_CONCAT(DISTINCT employee_id)
FROM employee
WHERE ROWNUM < 20';
execute immediate v_sql INTO v_some_string;
RETURN v_some_string;
END;