# Re: difference between UNION operator and OR in where clause

From: Brian Selzer <brian_at_selzer-software.com>

Date: Tue, 10 Jul 2007 17:39:28 GMT

Message-ID: <kvPki.45835$5j1.21632_at_newssvr21.news.prodigy.net>

>> news:1184083980.683470.307900_at_g4g2000hsf.googlegroups.com...

Date: Tue, 10 Jul 2007 17:39:28 GMT

Message-ID: <kvPki.45835$5j1.21632_at_newssvr21.news.prodigy.net>

"Mike" <gongweigang_at_gmail.com> wrote in message news:1184086648.109524.92680_at_q75g2000hsh.googlegroups.com...

> On Jul 10, 12:36 pm, "Brian Selzer" <b..._at_selzer-software.com> wrote:

*>> "Mike" <gongweig..._at_gmail.com> wrote in message**>>*>> news:1184083980.683470.307900_at_g4g2000hsf.googlegroups.com...

*>>**>>**>>**>>**>>**>> > 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.**>>**>> > but the where clause : "where publication.pubid = book.pubid or**>> > publication.pubid = journal.pubid"**>> > will be TRUE.**>>**>> > Similiar answer to the other part of your question.**>>**>> > Please correct me if I am wrong.**>>*

*>> PUB BOOK JOURNAL WHERE?*

*>> 1 1 6 TRUE*

*>> 1 1 7 TRUE*

*>> 1 1 8 TRUE*

*>> 1 1 9 TRUE*

*>> 1 1 10 TRUE*

*>> 2 2 6 TRUE*

*>> 2 2 7 TRUE*

*>> 2 2 8 TRUE*

*>> 2 2 9 TRUE*

*>> 2 2 10 TRUE*

*>> 6 1 6 TRUE*

*>> 6 2 6 TRUE**>> ... many many more combinations- Hide quoted text -**>>**>> - Show quoted text -*> > Use smaller tables: > publication: > --------------------------------------- > pubid title > 1 t-1 > 2 t-2 > 3 t-3 > -------------------------------------- > > book: > ------------------------------------- > pubid date > 1 d-1 > 2 d-2 > ------------------------------------ > > journal: > ------------------------------------- > pubid date > 3 d-3 > ------------------------------------ >

A table this small masks the problem.

If A and B are sets, then |A x B| = |A| * |B|. If |A| = 1, then |A x B| = |B|

> The cross product will be: > > publication.pubid publication.title book.pubid > book.date journal.pubic journal.date > WHERE > 1 t-1 > 1 d-1 > 3 d-3 TRUE > 1 t-1 > 2 d-2 > 3 d-3 FALSE > 2 t-2 > 1 d-1 > 3 d-3 FALSE > 2 t-2 > 2 d-2 > 3 d-3 TRUE > 3 t-3 > 1 d-1 > 3 d-3 TRUE > 3 t-3 > 2 d-2 > 3 d-3 FALSE >

As Lennart pointed out, this one should be TRUE.

> > Therefore, only 3 records are TRUE in the WHERE clause, ie. > > publication.pubid publication.title book.pubid > book.date journal.pubic journal.date > WHERE > 1 t-1 > 1 d-1 > 3 d-3 TRUE > 2 t-2 > 2 d-2 > 3 d-3 TRUE > 3 t-3 > 1 d-1 > 3 d-3 TRUE > > > then solution 1 and solution 2 give the same result. >Received on Tue Jul 10 2007 - 19:39:28 CEST