Re: difference between UNION operator and OR in where clause

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Tue, 10 Jul 2007 11:40:29 -0300
Message-ID: <46939a42$0$8866$9a566e8b@news.aliant.net>

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? Received on Tue Jul 10 2007 - 09:40:29 CDT