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,
3 Michael Stone
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:
- Every relational operation should yield a relation.
- 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 - 08:44:34 CDT