Using Oracle combine three tables to one with PIVOT - sql

Question

I have three Oracle SQL select queries which returns following results.
First select query returns result:
user_id | user_name |
---------|-----------|
1 | user_1 |
2 | user_2 |
3 | user_3 |
4 | user_4 |
second select query returns result:
exam_id | exam_name |
---------|-----------|
1 | exam_1 |
2 | exam_2 |
3 | exam_3 |
and the third select query returns result:
exam_id | user_id | exam_date |
---------|---------|-----------|
1 | 1 | 2017 |
1 | 2 | 2018 |
1 | 3 | 2017 |
2 | 3 | 2018 |
I would like to combine these queries to get result:
user_id | user_name | exam_1 | exam_2 | exam_3 |
---------|-----------|--------|--------|--------|
1 | user_1 | 2017 | | |
2 | user_2 | 2018 | | |
3 | user_3 | 2017 | 2018 | |
4 | user_4 | | | |
I would be grateful for any help?
Thank you #shrek for helping me out here. I managed to create the variable for pivot values but couldn't put the variable in the pivot. So I got help for that here and the final version (for 11g) looks like this:
variable x REFCURSOR
DECLARE
exam_ids VARCHAR2(255);
BEGIN
SELECT
LISTAGG(''''
|| exam_id
|| ''' AS "'
|| exam_name
|| '"',',') WITHIN GROUP(
ORDER BY
exam_id ASC
)
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';
END;
/
print x
So that works in the SQL Developer and SQL*Plus. But not when trying to use database with PHP file. For that I needed to create procedure which then could be called from PHP file. Here is problem when trying to use code above from PHP file and the resolution.

Answer 1

This should get you going -
CREATE TABLE users
(user_id varchar2(9), user_name varchar2(11))
;
INSERT ALL
INTO users (user_id, user_name)
VALUES ('1', 'user_1')
INTO users (user_id, user_name)
VALUES ('2', 'user_2')
INTO users (user_id, user_name)
VALUES ('3', 'user_3')
INTO users (user_id, user_name)
VALUES ('4', 'user_4')
SELECT * FROM dual
;
CREATE TABLE exam
(exam_id varchar2(9), exam_name varchar2(11))
;
INSERT ALL
INTO exam (exam_id, exam_name)
VALUES ('1', 'exam_1')
INTO exam (exam_id, exam_name)
VALUES ('2', 'exam_2')
INTO exam (exam_id, exam_name)
VALUES ('3', 'exam_3')
SELECT * FROM dual
;
CREATE TABLE exam_user
(exam_id varchar2(9), user_id varchar2(9), exam_date varchar2(11))
;
INSERT ALL
INTO exam_user (exam_id, user_id, exam_date)
VALUES ('1', '1', '2017')
INTO exam_user (exam_id, user_id, exam_date)
VALUES ('1', '2', '2018')
INTO exam_user (exam_id, user_id, exam_date)
VALUES ('1', '3', '2017')
INTO exam_user (exam_id, user_id, exam_date)
VALUES ('2', '3', '2018')
SELECT * FROM dual
;
Query -
SELECT * FROM (
SELECT U.USER_ID, U.USER_NAME, E.EXAM_NAME,EU.EXAM_DATE
FROM USERS U, EXAM 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_NAME IN ('exam_1' as exam_1, 'exam_2' as exam_2,'exam_3' as exam_3))
order by 1
;
Output -
USER_ID USER_NAME EXAM_1 EXAM_2 EXAM_3
1 user_1 2017 (null) (null)
2 user_2 2018 (null) (null)
3 user_3 2017 2018 (null)
4 user_4 (null) (null) (null)