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From: ecec@ur-tut.UUCP (Eric Carleen)
Newsgroups: net.database
Subject: Problem with relational theory
Message-ID: <595@ur-tut.UUCP>
Date: Thu, 14-Aug-86 12:34:20 EDT
Article-I.D.: ur-tut.595
Posted: Thu Aug 14 12:34:20 1986
Date-Received: Sun, 17-Aug-86 06:22:02 EDT
Organization: Univ. of Rochester Computing Center
Lines: 40
Keywords: relational, delete



I've found that apparently I have a severe misunderstanding of
the use of a relational data base, specifically Ingres.  I'm sending
out a summary of my question because I've found that a number of
other people have the same misunderstanding.

Suppose that I have 3 tables of names, call them table1, table2, and 
table3.  The names in tables 2 or 3 may or may not be in table1.  I
would like to delete all those records in table1 that are already listed
in the other two tables.

If I give the command:

	delete table1 where table1.name = table2.name
						or table1.name = table3.name

then I get what I was after: entries that match EITHER table2 OR table3
are deleted.

unless... table3 is empty.  If table3 is empty, then nothing is deleted
from table1, regardless of whether or not there are any matching entries
in table2.  (The work-around is obvious, but I thought unnecessary.)

Technical support at Relational Technologies insists that this is the
correct way for a deletion to occur.

I guess that I need to learn more about relational databases - so would
people please mail me names of references that are readable?  Thanks much.


I want to emphasize that I am generally very happy with Ingres, just
surprised at the behavior of this particular delete command.


                            Eric Carleen
                            University of Rochester Medical Center
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