SQL: How to find duplicates based on two fields?

Question

I have rows in an Oracle database table which should be unique for a combination of two fields but the unique constrain is not set up on the table so I need to find all rows which violate the constraint myself using SQL. Unfortunately my meager SQL skills aren't up to the task.

My table has three columns which are relevant: entity_id, station_id, and obs_year. For each row the combination of station_id and obs_year should be unique, and I want to find out if there are rows which violate this by flushing them out with an SQL query.

I have tried the following SQL (suggested by this previous question) but it doesn't work for me (I get ORA-00918 column ambiguously defined):

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 

Can someone suggest what I'm doing wrong, and/or how to solve this?

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