How to delete duplicate rows from a MySQL table

Question

I have a MySQL table like:

ID, Col1, Col2, Col3, Col4, etc...

ID is a primary key and has been working since the table's creation.

What I want to do is delete all but one records where all the other columns are identical.

Answer 1

DELETE DupRows.*
FROM MyTable AS DupRows
   INNER JOIN (
      SELECT MIN(ID) AS minId, col1, col2
      FROM MyTable
      GROUP BY col1, col2
      HAVING COUNT(*) > 1
   ) AS SaveRows ON SaveRows.col1 = DupRows.col1 AND SaveRows.col2 = DupRows.col2
      AND SaveRows.minId <> DupRows.ID;

Of course you have to extend col1, col2 in all three places to all columns.

Edit: I just pulled this out of a script I keep and re-tested, it executes in MySQL.

Answer 2

1. RENAME TABLE [table w/ duplicates] TO [temporary table name]

2. Create an identical table with the original table name which contained the duplicates.

3. INSERT INTO [new table] SELECT DISTINCT * FROM [old table with duplicates]

4. Delete the temporary tables.

Answer 3

Without nested selects or temporary tables.

DELETE  t1
FROM    table_name t1, table_name t2
WHERE   
            (t1.Col1 = t2.Col1 OR t1.Col1 IS NULL AND t2.Col1 IS NULL)
        AND (t1.Col2 = t2.Col2 OR t1.Col2 IS NULL AND t2.Col2 IS NULL)
        AND (t1.Col3 = t2.Col3 OR t1.Col3 IS NULL AND t2.Col3 IS NULL)
        AND (t1.Col4 = t2.Col4 OR t1.Col4 IS NULL AND t2.Col4 IS NULL)
        ...
        AND t1.ID < t2.ID;