Re: difference between UNION operator and OR in where clause

On Jul 10, 1:36 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
> Mike wrote:
> > On Jul 10, 11:18 am, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>
> >>Mike wrote:
>
> >>>On Jul 10, 10:40 am, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>
> >>>>Mike wrote:
>
> >>>>>On Jul 10, 9:59 am, David Portas
> >>>>><REMOVE_BEFORE_REPLYING_dpor..._at_acm.org> wrote:
>
> >>>>>>On 10 Jul, 14:39, Mike <gongweig..._at_gmail.com> wrote:
>
> >>>>>>>tables:
> >>>>>>>1. publication(pubid, title)
> >>>>>>>2. book(pubid, date)
> >>>>>>>3. journal(pubid, date)
>
> >>>>>>>purpose: find all publication titles for books or journals.
>
> >>>>>>>solution 1: use set "union"
>
> >>>>>>>select title
>
> >>>>>>>from publication, ( (select pubid from book) UNION (select pubid from
>
> >>>>>>>journal)) bj
> >>>>>>>where publication.pubid = bj.pubid
>
> >>>>>>>solution 2: use "or" operator in the where clause
>
> >>>>>>>select title
>
> >>>>>>>from publication, book, journal
>
> >>>>>>>where publication.pubid = book.pubid or publication.pubid =
> >>>>>>>journal.pubid
>
> >>>>>>>This example is taken from some lecture notes about sql. The notes
> >>>>>>>said the solution 2 was wrong.
>
> >>>>>>>Anyone knows why ?
>
> >>>>>>Let me pose two more questions for you. What are the keys for each
> >>>>>>table? What is the difference between ALL and DISTINCT?
>
> >>>>>>If you can answer those questions then you may be on the way to the
> >>>>>>answer you are looking for.
>
> >>>>>>--
> >>>>>>David Portas- Hide quoted text -
>
> >>>>>>- Show quoted text -
>
> >>>>>The keys for all the 3 tables are "pubid". "ALL" means to allow
> >>>>>duplicated records, and "DISTINCT" will remove the duplicated ones.
>
> >>>>>But I still don't get it.
>
> >>>>>Could you give more hints ?
>
> >>>>In the second case, if a book id matches a publication id, how many
> >>>>journals will match the WHERE clause? Remember TRUE OR FALSE = ____
> >>>>Similarly if a journal id matches the a publication id, how many books
> >>>>will match the WHERE clause? Remember FALSE OR TRUE = ____
>
> >>>>I will let you fill in the blanks.
>
> >>>>Consider as well the nullary cases. What happens to the result of the
>
> >>>>FROM clause if there are no books at all or if there are no journals at all?- Hide quoted text -
>
> >>>>- Show quoted text -
>
> >>>In the second case, if a book id matches a publication id, how many
> >>>journals will match the WHERE clause?
>
> >>>0
>
> >>>Similarly if a journal id matches the a publication id, how many books
> >>>will match the WHERE clause?
>
> >>>0
>
> >>>>Consider as well the nullary cases.
>
> >>>This is the case which fails the solution 2, I think.
>
> >>It is one case and one mechanism by which solution 2 fails. Go back
> >>where I left the blanks and fill them in. Then re-read the paragraph
> >>they appear in.- Hide quoted text -
>
> >>- Show quoted text -
>
> > My answer for your previous question.
>
> >>In the second case, if a book id matches a publication id, how many
>
> > journals will match the WHERE clause?
>
> > If a book id matches a publication id, 0 journal will match the WHERE
> > clause.
>
> I disagree. No journals will match "publication.pubid = journal.pubid",
> but that is only part of the WHERE clause. More than zero journals will
> match the where clause. How many?
>
> TRUE OR X = ____
>
> X OR TRUE = ____
both are TRUE.

>
> > but the where clause : "where publication.pubid = book.pubid or
> > publication.pubid = journal.pubid"
> > will be TRUE.
>
> Yes, it will be true. It will be true for how many journals?

It will be true for |book.pubid|.

>
> > Similiar answer to the other part of your question.
>
> > Please correct me if I am wrong.
>
> You are focusing on single records rather than the CROSS PRODUCT, but
> the FROM clause yields a CROSS PRODUCT.
>
> If you have 5 publications P, 3 books B and 2 journals J:
>
> P has 5 rows
> B has 3 rows
> J has 2 rows
> P X B has 15 rows
> P X J has 10 rows
> B X J has 6 rows
> P X B X J has 30 rows
>
> Let's take P X B and restrict it to rows with equal pubid (ie. replace
> CROSS PRODUCT with NATURAL JOIN) and call the result PB.
>
> PB has 3 rows
> PB X J has ____ rows ?- Hide quoted text -

Since |PB| = 3 and |J|=2, |PB * J| = 6.

>
> - Show quoted text -
Received on Tue Jul 10 2007 - 21:19:10 CEST