[vox-tech] A complicated SQL question

[Ken Bloom]

On Thu, Nov 09, 2006 at 12:49:25PM -0800, Richard S. Crawford wrote:
> I have a number of rows in my database which look something like this:
> 
> 
> id | username | canonicalname | password
> ---+----------+---------------+-----------------------
> 01 | asmith   | ART SMITH     | md5 encrypted password
> 02 | asmith1  | ART SMITH     | <NULL>
> 03 | bjones   | BILL JONES    | md5 encrypted password
> 04 | bjones12 | BILL JONES    | <NULL>
> ---+----------+---------------+-----------------------
> 
> 
> There are something like 275 entries like these.  I'm having a hell of a time 
> finding a way to select rows like 02 and 04, where canonicalname is a 
> duplicate of the same value in another row and the password is blank, and 
> then deleting those rows.  After executing my query, the table should look 
> like this:
> 
> 
> id | username | canonicalname | password
> ---+----------+---------------+-----------------------
> 01 | asmith   | ART SMITH     | md5 encrypted password
> 03 | bjones   | BILL JONES    | md5 encrypted password
> ---+----------+---------------+-----------------------
> 
> 
> Is there a simple way to execute a single select query which would pull up all 
> the rows I need?

Not in MySQL. It takes 2 (well technically 3) SQL statements;

create temporary table todelete
select a.id as id from THETABLE a, THETABLE b where
a.username=b.username AND a.canonicalname=b.canoncialname and
a.password is null and b.password is not null;

delete from THETABLE using todelete, THETABLE where
   todelete.id=THETABLE.id;

drop temporary table todelete;

Maybe other DB's are more intelligent about self-joins in deletes, in
which case you can probably figure out how to combine all 3 of these
based on what I've posted.

--Ken

-- 
Ken Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu/~kbloom1/
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