How can I delete one of two perfectly identical rows?

Note that MySQL has its own extension of DELETE, which is DELETE ... LIMIT, which works in the usual way you'd expect from LIMIT: http://dev.mysql.com/doc/refman/5.0/en/delete.html

The MySQL-specific LIMIT row_count option to DELETE tells the server the maximum number of rows to be deleted before control is returned to the client. This can be used to ensure that a given DELETE statement does not take too much time. You can simply repeat the DELETE statement until the number of affected rows is less than the LIMIT value.

Therefore, you could use DELETE FROM some_table WHERE x="y" AND foo="bar" LIMIT 1; note that there isn't a simple way to say "delete everything except one" - just keep checking whether you still have row duplicates.

One option to solve your problem is to create a new table with the same schema, and then do:

INSERT INTO new_table (SELECT DISTINCT * FROM old_table)

and then just rename the tables.

You will of course need approximately the same amount of space as your table requires spare on your disk to do this!

It's not efficient, but it's incredibly simple.

SET ROWCOUNT 1
DELETE FROM [table] WHERE ....
SET ROWCOUNT 0

This will only delete one of the two identical rows