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Relational Algebra Expression

From: gamehack <gamehack_at_gmail.com>
Date: Sat, 9 Feb 2008 08:32:00 -0800 (PST)
Message-ID: <e929bb2f-07e8-4fa6-b6f3-c30a83f25832_at_l1g2000hsa.googlegroups.com> 





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 - 17:32:00 CET












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