remove rows with some duplicate column value

Question

Suppose I have a table with column A like following :

a  
--
x  
y  
m  
x  
n  
y  

I want to delete all rows that have duplicate a column value and keep just one value. After this operation, my column would be like If you do :

select distinct a from A;

I know how to select rows with repeated a column values But I can't just replace select with DELETE because it would delete the unique values too.
Any help would be greatly appreciated.

Answer 1

In Oracle, you can do this by using the hidden column rowid and a correlated subquery:

delete from a
    where rowid > (select min(rowid)
                   from a a2
                   where a.a = a2.a
                  );

Alternatively, you can phrase this as a not in:

delete from a
    where rowid not in (select min(rowid)
                        from a a2
                        group by a2.a
                       );

Answer 2

You can use combination of CTE and Ranking function

;With cte As
(
Select ROW_NUMBER() OVER (PARTITION BY colA ORDER BY colA) as rNum
From yourTable 
)

Delete From cte
Where rNum<>1

Answer 3

In SQL, You can use CTE and delete the duplicated rows. See the query below.

WITH CTE AS(
   SELECT a,
       RN = ROW_NUMBER()OVER(PARTITION BY a ORDER BY a)
   FROM A
)
DELETE FROM CTE WHERE RN > 1