使用oracle在外连接中只获得一行

WITH
  x AS( SELECT 1 id FROM dual UNION SELECT 2 FROM dual UNION SELECT 3 FROM dual ),
  y AS( SELECT 1 id, 1 nr, 'B' code FROM dual
        UNION SELECT 1, 2, 'A'  FROM dual
        UNION SELECT 2, 2, 'A' FROM dual
) -- end of test data
SELECT  *
FROM    (
        SELECT  x.id, y.nr, y.code, ROW_NUMBER() OVER (PARTITION BY x.id ORDER BY y.nr) AS rn
        FROM    x
        LEFT OUTER JOIN y
        ON   y.id = x.id
)
WHERE rn = 1