Delete all Duplicate Rows except for One in MySQL?

If you want to keep the row with the lowest id value:

DELETE FROM NAMES
 WHERE id NOT IN (SELECT * 
                    FROM (SELECT MIN(n.id)
                            FROM NAMES n
                        GROUP BY n.name) x)

If you want the id value that is the highest:

DELETE FROM NAMES
 WHERE id NOT IN (SELECT * 
                    FROM (SELECT MAX(n.id)
                            FROM NAMES n
                        GROUP BY n.name) x)

The subquery in a subquery is necessary for MySQL, or you'll get a 1093 error.

Editor warning: This solution is computationally inefficient and may bring down your connection for a large table.

NB - You need to do this first on a test copy of your table!

When I did it, I found that unless I also included AND n1.id <> n2.id, it deleted every row in the table.

1. If you want to keep the row with the lowest id value:

   DELETE n1 FROM names n1, names n2 WHERE n1.id > n2.id AND n1.name = n2.name
   

2. If you want to keep the row with the highest id value:

   DELETE n1 FROM names n1, names n2 WHERE n1.id < n2.id AND n1.name = n2.name
   

I used this method in MySQL 5.1

Not sure about other versions.

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Update: Since people Googling for removing duplicates end up here
Although the OP's question is about DELETE, please be advised that using INSERT and DISTINCT is much faster. For a database with 8 million rows, the below query took 13 minutes, while using DELETE, it took more than 2 hours and yet didn't complete.

INSERT INTO tempTableName(cellId,attributeId,entityRowId,value)
    SELECT DISTINCT cellId,attributeId,entityRowId,value
    FROM tableName;