[TriLUG] OT: SQL "Group by" question

[Cristobal Palmer]

You /have/ to have the column you want to group by in the select
statement. How can you group by values in a column that you didn't
retrieve? You have to have those values.

The prof in my database class beat us over the head with this one. If
you want a more detailed answer I can dig up the class notes on that.

-CMP

On 10/3/06, Andrew Perrin <clists at perrin.socsci.unc.edu> wrote:
> Sorry for the slightly OT post, but my soft-science mind is having trouble
> wrapping itself around an SQL problem.
>
> I have a table of book-review assignments, including a unique identifier
> (assign_id); the book's isbn (isbn); and the date we made the assignment
> (assign_date). There are often numerous assignments per isbn. I would like
> to retrieve the assign_id of the *latest* assignment per isbn.  I can get
> this far:
>
> select assign.isbn, max(assign.assign_date) as last_assign_date from
> assign group by assign.isbn;
>
> ...which will give me the isbn and latest assign date, but of course I
> can't just ask for the assign_id in the way that seems obvious to me:
>
> select assign.assign_date, assign.isbn, max(assign.assign_date) as
> last_assign_date from assign group by assign.isbn;
>
> because assign_date isn't in the GROUP BY clause.
>
> I can't imagine there isn't an accepted answer to this - any advice? I am,
> by the way, using postgresql 8.1 on debian linux.
>
> Thanks,
> Andy
>
> ----------------------------------------------------------------------
> Andrew J Perrin - andrew_perrin (at) unc.edu - http://perrin.socsci.unc.edu
> Assistant Professor of Sociology; Book Review Editor, _Social Forces_
> University of North Carolina - CB#3210, Chapel Hill, NC 27599-3210 USA
> New Book: http://www.press.uchicago.edu/cgi-bin/hfs.cgi/00/178592.ctl
>
>
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-- 
Cristobal M. Palmer
UNC-CH SILS Student -- ils.unc.edu/~cmpalmer
TriLUG Vice Chair
"There are many roads to enlightenment, and thus many roads back to
the One True Debian" --crimsun