On Fri, 26 Apr 2013 11:01:28 +0200 CR Lender <crlender@xxxxxxxxx> wrote: > I have two tables with countries and persons living in those > countries: > > create table countries ( > code char(2) not null primary key, Isn't this redundant? Primary keys are always NOT NULL. > Enter a third table for loans that can only be made between persons > living in EU countries: > > create table eu_loans ( > donor text not null references persons(name), > recipient text not null references persons(name), > primary key (donor, recipient) > ); Side question - are you really limiting them to one loan each? Can't a donor have two active loans with the same recipient? > I can add a trigger on eu_loans to check if Diane and Betty both live > in the EU. The problem is how to prevent one of them from moving to a > non-EU country (if they do, the loan has to be cancelled first). They > are however allowed to move to other EU countries. Wouldn't two constraints, one for each of donor and recipient, do the job? Moving a person out of the EU would have the same effect as deleting them. The constraint would prevent it. -- D'Arcy J.M. Cain <darcy@xxxxxxxxx> | Democracy is three wolves http://www.druid.net/darcy/ | and a sheep voting on +1 416 788 2246 (DoD#0082) (eNTP) | what's for dinner. IM: darcy@xxxxxxx, VOIP: sip:darcy@xxxxxxx -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
