SQL:如何根据两个字段查找重复项？

Question

我在Oracle数据库表中有行,对于两个字段的组合应该是唯一的,但是没有在表上设置唯一约束,所以我需要使用SQL找到所有违反约束的行.不幸的是,我的微薄SQL技能无法胜任.

我的表有三列相关:entity_id,station_id和obs_year.对于每一行,station_id和obs_year的组合应该是唯一的,我想通过SQL查询将它们刷出来查明是否存在违反此行的行.

我尝试了以下SQL(由前一个问题提出),但它对我不起作用(我得到ORA-00918列模糊定义):

SELECT
entity_id, station_id, obs_year
FROM
mytable t1
INNER JOIN (
SELECT entity_id, station_id, obs_year FROM mytable 
GROUP BY entity_id, station_id, obs_year HAVING COUNT(*) > 1) dupes 
ON 
t1.station_id = dupes.station_id AND
t1.obs_year = dupes.obs_year

有人可以建议我做错了什么,和/或如何解决这个问题？

Answer 1

SELECT  *
FROM    (
        SELECT  t.*, ROW_NUMBER() OVER (PARTITION BY station_id, obs_year ORDER BY entity_id) AS rn
        FROM    mytable t
        )
WHERE   rn > 1

Answer 2

SELECT entity_id, station_id, obs_year
FROM mytable t1
WHERE EXISTS (SELECT 1 from mytable t2 Where
       t1.station_id = t2.station_id
       AND t1.obs_year = t2.obs_year
       AND t1.RowId <> t2.RowId)