[vox-tech] SQL help - unions

[Michael Wenk]

On Tuesday 20 July 2004 05:49 pm, Michael J Wenk wrote:
> On Tue, Jul 20, 2004 at 04:44:47PM -0700, Peter Jay Salzman wrote:
> > Total newbie here.  I know less about SQL than I do about xkb.   ;-)
> >
> > I have two tables, with the same structure.  Call them [Fall 2003] and
> > [Spring 2004].  They each hold data about enrolled students.  One of the
> > fields of these tables is SSN (social security number).
> >
> > I can get an unduplicated headcount for the academic year with:
> >
> >    SELECT SSN from [Fall 2003]
> >    UNION
> >    SELECT SSN from [Spring 2004];
> >
> > There's also a field called "race".  If the race field is 0, the student
> > is African American.
> >
> > How would I modify the above SQL statement to list the unduplicated
> > headcount of all African Americans?  I would like to do something like:
> >
> >    SELECT SSN from [Fall 2003]
> >    UNION
> >    SELECT SSN from [Spring 2004]
> >    and race == 1;
> >
> > but of course that doesn't work.  :)
> >
> > Thanks!
> > Pete
>
> If memory serves, you want:

Well apparently, it doesn't serve me tonite.  At least not well. 
>
> SELECT distinct SSN from Fall_2003, Spring_2004 where race = 0
>

Unfortunately, a join(as above) will join all the columns of one table to 
another based on the join condition, and is not what you want.

You can use a series of joins and an extra table to do this, but in my view
its kinda cryptic, and using a union makes more sense.  

Mike
-- 
wenk at praxis.homedns.org
Mike Wenk