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.

Patrick Nelson <pnelson@neatech.com> writes:

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;

Seems messy. Why not

UPDATE db1
SET dat = (SELECT MIN(b.dat) FROM db2 b WHERE db1.id = b.id);

I think this is actually SQL-standard, as well as being more readable;
the form with FROM is definitely not standard.

regards, tom lane