Re: Relational Algebra Expression

gamehack wrote:

> Hi,
>
> I've been trying to figure out how to use relational algebra to
> extract some data from the following relation:
> <Year, Name, Votes> where the primary key is Year+Name (composite
> key). A sample table could contain the following entries:
> 1999 | Joe | 34
> 1999 | James | 45
> 2000 | Mike | 24
> 2001 | George | 17
> 2001 | Josh | 69
>
> 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.
>
> Thanks,
> g

Equijoin on year and inequality theta-join on name project on year. Received on Sat Feb 09 2008 - 17:47:26 CET