Oracle SQL：将行转换为多列

Question

我正在使用 Oracle 11G，需要一种方法在 select 语句中将行转换为新的列组。我们正在将一些数据转换为 1:3 关系，并且需要一种方法将其放入视图中。您能帮我们转换如下所示的数据吗：

+---------+------------+
| User_Id | Station_Id |  
+---------+------------+  
|       1 |        203 |
|       1 |        204 |
|       2 |        203 |
|       3 |        487 |
|       3 |       3787 |
|       3 |        738 |
+---------+------------+

进入这个：

+---------+-------------+-------------+---------------+
| User_Id | Station_One | Station_Two | Station_Three |
+---------+-------------+-------------+---------------+
|       1 |         203 | 204         | Null          |
|       2 |         203 | Null        | Null          |
|       3 |         487 | 3787        | 738           |
+---------+-------------+-------------+---------------+

让我知道您想要的其他细节，并感谢您提供的任何帮助！

Answer 1

您可以使用row_number 和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

见the demo

您也可以使用row_number 和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

见the demo

Answer 2

根据我的经验，最简单的方法是使用条件聚合：

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;

只需将上述查询中的mydata CTE 替换为您的表名即可：

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;