Using Oracle: Can I use 'dynamically' created variable in the pivot clause? - oracle

Question

I'm trying to create pivot clause that gets it parameters from variable.
I have some test code like:
SELECT * FROM (
SELECT U.USER_ID, U.USER_NAME, E.EXAM_ID, EU.EXAM_DATE
FROM USERS U, EXAMS E, EXAM_USER EU
WHERE U.USER_ID = EU.USER_ID(+)
AND E.EXAM_ID(+) = EU.EXAM_ID
ORDER BY U.USER_ID
)
PIVOT (MAX(EXAM_DATE) FOR EXAM_ID IN ('3' AS "exam 3",'2' AS "exam 2",'1' AS "exam 1"))
order by 1
;
This works just fine. Then I declared the variable EXAM_IDS like:
DECLARE
EXAM_IDS VARCHAR2 (255);
BEGIN
SELECT LISTAGG('''' || EXAM_ID || ''' AS "' || EXAM_NAME || '"', ',')
WITHIN GROUP (ORDER BY EXAM_ID DESC)
INTO EXAM_IDS
FROM EXAMS;
END;
I'm pretty sure the variable EXAM_IDS has now a string as used in pivot clause(?) but I don know how to combine these two:
DECLARE
EXAM_IDS VARCHAR2 (255);
BEGIN
SELECT LISTAGG('''' || EXAM_ID || ''' AS "' || EXAM_NAME || '"', ',')
WITHIN GROUP (ORDER BY EXAM_ID DESC)
INTO EXAM_IDS
FROM EXAMS;
END;
SELECT * FROM (
SELECT U.USER_ID, U.USER_NAME, E.EXAM_ID, EU.EXAM_DATE
FROM USERS U, EXAMS E, EXAM_USER EU
WHERE U.USER_ID = EU.USER_ID(+)
AND E.EXAM_ID(+) = EU.EXAM_ID
ORDER BY U.USER_ID
)
PIVOT (MAX(EXAM_DATE) FOR EXAM_ID IN (' || EXAM_IDS || '))
order by 1
;
And this does not work. Is there a way to do this or should I just run two separate SQL queries?
More info about this setup (like my classes) can be found from Using Oracle combine three tables to one with PIVOT

Answer 1

For 12c and above, you may use DBMS_SQL.RETURN_RESULT by opening a REFCURSOR for the dynamic PIVOT query.
I have removed the notorious (+) syntax for left join, always use the ANSI join syntax.
DECLARE
exam_ids VARCHAR2(255);
x SYS_REFCURSOR;
BEGIN
SELECT
LISTAGG(''''
|| exam_id
|| ''' AS "'
|| exam_name
|| '"',',') WITHIN GROUP(
ORDER BY
exam_id DESC
)
INTO exam_ids
FROM
exam;
OPEN x FOR 'SELECT
*
FROM
(
SELECT
u.user_id,
u.user_name,
e.exam_id,
eu.exam_date
FROM
users u
LEFT JOIN exam_user eu ON u.user_id = eu.user_id
LEFT JOIN exam e ON e.exam_id = eu.exam_id
ORDER BY
u.user_id
)
PIVOT ( MAX ( exam_date )
FOR exam_id
IN ( ' || EXAM_IDS || ' )
)
ORDER BY
1';
dbms_sql.return_result(x);
END;
/
For 11g, you may use a bind variable and print command ( works in sqlplus and in sql developer/Toad when run as script (F5))
variable x REFCURSOR -- bind variable declared.
DECLARE
.. -- no need to declare sys_refcursor
BEGIN
..
OPEN :x FOR 'SELECT . --note the change with colon
*
FROM
(
SELECT
..
END;
/
print x