How do I delete duplicate rows and keep the first row?

Question

I made a mistake and I have unwanted duplicates.

I have a table with 4 key fields. A1, k1, k2, k3.

A1 is auto increment and the primary key.

the combination of k1, k2 and k3 is supposed to be unique and I have to delete the duplicate rows before I create a unique index. Some rows have one duplicate, some have many.

SELECT CONCAT(k1, k2, k) AS dup_value
  FROM myviews
 GROUP BY dup_value
HAVING (COUNT(dup_value) > 1)

shows me duplicates values that I need to deal with. But now I don't know how to keep one and delete the rest of each duplicate set.

Answer 1

Backup your data, then...

MySQL supports JOINs in DELETE statements. If you want to keep the first of the duplicates:

DELETE a
  FROM MYVIEWS a
  JOIN (SELECT MIN(t.a1) AS min_a1, t.k1, t.k2, t.k3
          FROM MYVIEWS t
      GROUP BY t.k1, t.k2, t.k3
        HAVING COUNT(*) > 1) b ON b.k1 = a.k1
                              AND b.k2 = a.k2
                              AND b.k3 = a.k3
                              AND b.min_a1 != a.a1

If you want to keep the last of the duplicates:

DELETE a
  FROM MYVIEWS a
  JOIN (SELECT MAX(t.a1) AS max_a1, t.k1, t.k2, t.k3
          FROM MYVIEWS t
      GROUP BY t.k1, t.k2, t.k3
        HAVING COUNT(*) > 1) b ON b.k1 = a.k1
                              AND b.k2 = a.k2
                              AND b.k3 = a.k3
                              AND b.max_a1 != a.a1

Answer 2

You can create a new table with the same structure but empty, then create the unique key on it, then do a INSERT IGNORE / SELECT * FROM the original table into the new table, then delete the original table.

INSERT IGNORE will automatically ignore any primary or unique key issues and just skip the duplicates.