Relational Algebra Expression

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 Received on Sat Feb 09 2008 - 10:32:00 CST