you might have hit: http://bugs.mysql.com/bug.php?id=58081
2012/7/5 Halász Sándor <hsv@stripped>
> Here is something queer:
> select ifnull(email, round(10000 * rand(), 1)) as ux, count(*) from nam
> group by ux;
> ERROR 1062 (23000): Duplicate entry '2514.0' for key 'group_key'
> I have a name-list, with e-mail address or not. I wanted to fill the NULL
> e-mail addresses with something random, and, I hope, unique. (ROUND is
> there only to make that surprising error liklier.)
> The field "email" is not UNIQUE, not a key, no intention of making it such.
> Why this error? Is RAND called more than once for each record?
> I tryed also UUID, but that came with its own shortcoming: if the UUID
> call were the whole field, it indeed was once called for every record, but
> if argument to IFNULL, only once for the whole query.
> Version 5.5.8
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