Re: Relational Algebra Expression

On Feb 9, 4:59 pm, gamehack <gameh..._at_gmail.com> wrote:
> On Feb 9, 4:47 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
>
> > gamehack wrote:
> > [snip]
> > > What I'm trying to do is extract all the years where we have more than
> > > 1 relation for the year. For the sample table, we need to get:
> > > 1999
> > > 2001
>
> > > I tried to do in a couple of ways but I couldn't. I tried using a
> > > projection on Year so that I can remove duplicates but then I can't
> > > just use difference because the new relations are not compatible. Any
> > > hints are greatly appreciated.
>
> > Equijoin on year and inequality theta-join on name project on year.
>
> Hi,
>
> I can't really understand what this means - can you bracket it so I
> can see the results of each operation?
>
> Thanks very much,
> g

Well Bob gave you everything you needed, but I guess you're learning the stuff at the moment (coursework?) so lets break down his instructions:

1. EQUIJOIN R with itself (renamed B) where Year = B.Year
2. RESTRICT where Name != B.Name
3. PROJECT on Year

Note if you use SQL it can allow duplicates (which is of course particularly brain-dead given a relation is a set), so you have to specify you want distinct tuples:

SELECT DISTINCT Year FROM R, R as B
WHERE R.Year = B.Year AND R.Name != B.Name Received on Sat Feb 09 2008 - 23:16:38 CET