Question

I have two tables (id_test, test) , each of them has an ID column, which is unique, and two entries with the same id in the two tables are the same. Now, i have another column in one of the tables (id_test) that also should be unique, so I want to eliminate duplicates according to this other column, let’s call it YD.

To identify the duplicates I used

SELECT ID, YD AS x, COUNT(*) AS y
FROM id_test 
GROUP BY x 
HAVING y>1;

now, I want to delete these entries in both tables. How can I do it?

Answer 1

This query shows the first ID for every YD in id_test table:

SELECT ID, YD
FROM id_test
GROUP BY YD

and these are the rows you have to keep. The following query returns the IDs you have to delete:

SELECT id_test.ID
FROM id_test LEFT JOIN (select ID, YD from id_test group by YD) id_test_keep
     on id_test.ID=id_test_keep.ID and id_test.YD = id_test_keep.YD
WHERE id_test_keep.ID IS NULL

Now I think i need more details about your tables, but what I think you need is this:

DELETE FROM test
WHERE
  test.ID IN (
    SELECT id_test.ID
    FROM id_test LEFT JOIN (select ID, YD from id_test group by YD) id_test_keep
         on id_test.ID=id_test_keep.ID and id_test.YD = id_test_keep.YD
    WHERE id_test_keep.ID IS NULL)