Oracle SQL: Transform rows to multiple columns - sql

Question

I'm using Oracle 11G and need a way to turn rows into new groups of columns in a select statement. We're transitioning to a 1:3 relationship for some of our data and need a way to get it into a view. Can you help us transform data that looks like this:
+---------+------------+
| User_Id | Station_Id |
+---------+------------+
| 1 | 203 |
| 1 | 204 |
| 2 | 203 |
| 3 | 487 |
| 3 | 3787 |
| 3 | 738 |
+---------+------------+
into this:
+---------+-------------+-------------+---------------+
| User_Id | Station_One | Station_Two | Station_Three |
+---------+-------------+-------------+---------------+
| 1 | 203 | 204 | Null |
| 2 | 203 | Null | Null |
| 3 | 487 | 3787 | 738 |
+---------+-------------+-------------+---------------+
Let me know what ever other specifics you would like and thank you for any help you can give!

Answer 1

You can use row_number and self joins:
with cte as
(
select userid, stationid,
row_number() over(partition by userid order by stationid) rn
from tbl
)
select distinct c1.userid,
c1.stationid station_one,
c2.stationid station_two,
c3.stationid station_three
from cte c1
left join cte c2 on c1.userid=c2.userid and c2.rn=2
left join cte c3 on c1.userid=c3.userid and c3.rn=3
where c1.rn=1
See the demo
You can also do it with row_number and subqueries:
with cte as
(
select userid, stationid,
row_number() over(partition by userid order by stationid) rn
from tbl
)
select distinct userid,
(select stationid from cte c where c.userid=cte.userid and c.rn=1) station_one,
(select stationid from cte c where c.userid=cte.userid and c.rn=2) station_two,
(select stationid from cte c where c.userid=cte.userid and c.rn=3) station_three
from cte
See the demo

Answer 2

The easiest way to accomplish this in my experience is to use conditional aggregation:
WITH mydata AS (
SELECT 1 AS user_id, 203 AS station_id FROM dual
UNION ALL
SELECT 1 AS user_id, 204 AS station_id FROM dual
UNION ALL
SELECT 2 AS user_id, 203 AS station_id FROM dual
UNION ALL
SELECT 3 AS user_id, 487 AS station_id FROM dual
UNION ALL
SELECT 3 AS user_id, 3787 AS station_id FROM dual
UNION ALL
SELECT 3 AS user_id, 738 AS station_id FROM dual
)
SELECT user_id
, MAX(CASE WHEN rn = 1 THEN station_id END) AS station_one
, MAX(CASE WHEN rn = 2 THEN station_id END) AS station_two
, MAX(CASE WHEN rn = 3 THEN station_id END) AS station_three
FROM (
SELECT user_id, station_id, ROW_NUMBER() OVER ( PARTITION BY user_id ORDER BY rownum ) AS rn
FROM mydata
) GROUP BY user_id;
Just replace the mydata CTE in the above query with whatever your table's name is:
SELECT user_id
, MAX(CASE WHEN rn = 1 THEN station_id END) AS station_one
, MAX(CASE WHEN rn = 2 THEN station_id END) AS station_two
, MAX(CASE WHEN rn = 3 THEN station_id END) AS station_three
FROM (
SELECT user_id, station_id, ROW_NUMBER() OVER ( PARTITION BY user_id ORDER BY rownum ) AS rn
FROM mytable
) GROUP BY user_id;