references definition to multi-field primary key

Hi all,

I've got some fuel pumps, each of which sell a number of fuel grades - one
nozzle per grade, each nozzle connected to a specified tank containing that
grade.

I can define the tanks, the pump numbers, and the pump grade/nozzle config
using the tables below.

create table grades ( -- different types of fuel sold
gid character,
gdesc varchar(20),
gprice float,
primary key (gid)
);

create table tanks ( -- storage tanks
tid int4 not null,
tgrade character references grades(gid),
primary key (tid)
);

create table pumps ( -- list of pumps
pid int4 not null,
primary key (pid)
);

create table pgrades ( -- list of nozzles/grades per pump
pgpid int4 not null references pumps(pid),
pgnozzle int4 not null,
pgtank int4 not null references tanks(tid),
primary key (pgpid, pgseq)
);

My problem is that I want to be able to define a 'Pump Readings' table to show
per pump/nozzle the opening and closing reading. However, my problem is that
I don't know how to define the references so that I can only create a
preadings row for an existing pgrages entry. Here's the table less the
required references entry.

create table preadings ( -- daily reading per pump/nozzle
prdate date not null,
prpump int4 not null
prnozzle int4,
propen integer,
prclose integer,
primary key (prdate, prpump, prseq)
);

I only want the insert to work if prpid matches pgpid and prnozzle matches
pgnozzle.

--
Gary Stainburn

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