Re: Updating based on a join

On Mon, 21 Oct 2002, Patrick Nelson wrote:

> db1 (one)
>  id varchar(5)
>  dat varchar(8)
>
> db2 (many)
>  id varchar(5)
>  dat varchar(8)
>
> My update is:
>
> UPDATE db1
>  SET dat=(SELECT MIN(dat) FROM db2 WHERE id='A')
>  WHERE id='A';
>
> Which does exactly one record in db1 and I would like them all done.  So I
> used the FROM clause like:
>
> UPDATE db1
>  SET dat=c.dat
>  FROM (SELECT DISTINCT ON (b.id) a.sym, b.dat FROM db1 a, db2 b WHERE
> a.id=b.id) AS c
>  WHERE db1.id=c.id;
>
> Which seemed to have worked, but I'm not sure I've covered always getting
> the min value of multiple db2.dat for a given db1.id although my sub seems
> to have produced that ends I'm not confident.  Any comments?  Anecdotes?

I believe you need an order by in order to guarantee the minimum value.
order by b.id, b.dat would probably do it.