From: Wolfgang Keller <feliphil@gmx.net>
To: pgsql-sql@postgresql.org
Subject: Correct implementation of 1:n relationship with n>0?
Date: Tue, 30 Apr 2013 16:39:05 +0200
Message-ID: <20130430163905.77d43412c0176c3d9ebd8d90@gmx.net> (raw)
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It hit me today that a 1:n relationship can't be implemented just by a
single foreign key constraint if n>0. I must have been sleeping very
deeply not to notice this.
E.g. if there is a table "list" and another table "list_item" and the
relationship can be described as "every list has at least one
list_item" (and every list_item can only be part of one list, but
this is trivial).
A "correct" solution would require (at least?):
1. A foreign key pointing from each list_item to its list
2. Another foreign key pointing from each list to one of its list_item.
But this must be a list_item that itself points to the same list, so
just a simple foreign key constraint doesn't do it.
3. When a list has more than one list_item, and you want to delete the
list_item that its list points to, you have to "re-point" the foreign
key constraint on the list first. Do I need to use stored proceures
then for all insert, update, delete actions?
(4. Anything else that I've not seen?)
Is there a "straight" (and tested) solution for this in PostgreSQL, that
someone has already implemented and that can be re-used?
No, I definitely don't want to get into programming PL/PgSQL myself.
especially if the solution has to warrant data integrity under all
circumstances. Such as concurrent update, insert, delete etc.
TIA,
Sincerely,
Wolfgang
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Subject: Re: Correct implementation of 1:n relationship with n>0?
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