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The Relational Model & Queries That Naturally Return Duplicate Rows

From: Seun Osewa <seun.osewa_at_gmail.com>
Date: Fri, 8 Oct 2010 06:44:34 -0700 (PDT)
Message-ID: <0d9300b0-88be-40f3-ae94-97d1beac56dd_at_j18g2000yqd.googlegroups.com> 





Hi,



It's commonly understood that in the relational model:



    
  
  1.   Every relational operation should yield a relation.
  
  2.   Relations, being sets, cannot contain duplicate rows.






Imagine a 'USERS' relation that contains the following data.



ID FIRST_NAME LAST_NAME


 1 Mark       Stone
 2 Jane       Stone




 3 Michael    Stone



If someone runs the query "select LAST_NAME from USERS", a typical
database will return:



LAST_NAME



Stone


Stone


Stone



Since this is not a relation - because it contains duplicate rows -
what should an ideal RDBMS return?



Regards.
Received on Fri Oct 08 2010 - 15:44:34 CEST












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