Multi-column unique constraints with condition - postgresql

Question

Based on this example:
create table t (
id serial primary key,
a int not null,
b int default null
c int not null
);
Is there a way in PostgreSQL, without resorting to triggers, to incorporate a unique constraint that if there is already a record with values for a and c while b is null, no further records may be inserted in the same combination of a and c. The same should applied the other way round, if there is already a data set for a, b and c with concrete values, that a record with the same a and c may never be inserted where b is null, only further variants with different values for b.
examples:
(5, null, 1)
(4, 6, 9)
(4, 7, 9)
(5, null, 2)
(5, 3, 1) -- refused / error
(4, null, 9) -- also refused
—
Here is my specific use case to better illustrate the problem:
create table if not exists booking (
id serial4,
media_id int4 not null,
data_access int2 not null default session_user_id(),
product_line text not null,
product_variant text default null,
area_nr int2 not null,
net int2 not null,
period int2range not null,
-- If the start and end calendar weeks match, only one week is booked.
entry_time timestamp not null default now(),
constraint "internally used surrogate key to a booking" primary key (id),
constraint "no overlapping booking periods of a distributer" exclude using gist (
data_access with =,
product_line with =,
area_nr with =,
net with =,
period with &&
) where (product_variant is null),
constraint "no overlapping booking period of a sub-distributer" exclude using gist (
data_access with =,
product_line with =,
product_variant with =,
area_nr with =,
net with =,
period with &&
),
/*
constraint "known booked product"
foreign key (data_access, product_line, product_variant)
references product(data_access, line_abbr, variant_abbr),
constraint "known booked area"
foreign key (data_access, area_nr) references area(data_access, nr),
constraint "known booked net"
foreign key (data_access, product_line, net)
references net(data_access, product_line, nr),
constraint "known media of a booking"
foreign key (media_id) references media(id) on delete cascade,
*/
constraint "closed booking periods of areas" check (
upper_inc(period)
)
);
I commented out the foreign keys and only left them in for better understanding.
insert into booking
(product_line, product_variant, area_nr, net, period, media_id)
values
('CB', null, 1, 4, '[2239, 2245]', 5);
insert into booking
(product_line, product_variant, area_nr, net, period, media_id)
values
('CB', 'P', 1, 4, '[2239, 2245]', 5);
The last insert should result in an error since the entire distributor has already been booked for the given period.
insert into booking
(product_line, product_variant, area_nr, net, period, media_id)
values
('CB', 'F', 2, 6, '[2230, 2245]', 5);
insert into booking
(product_line, product_variant, area_nr, net, period, media_id)
values
('CB', 'P', 2, 6, '[2230, 2245]', 5);
insert into booking
(product_line, product_variant, area_nr, net, period, media_id)
values
('CB', null, 2, 6, '[2230, 2245]', 5);
The last one should give an error too since one or more sub-distributors have already been booked, so the entire distributor may no longer be available.
The problem: You first have to check whether a distributor or sub-distributor has already been booked in the same period / range for the same area, net and products, which I solve using a trigger. (due to the cool range feature and related constraints, I was already able to very elegantly exclude overlapping booking periods without a trigger):
create or replace function _avoid_incomplete_distributers() returns trigger as $$ begin
if exists (select from booking where
product_line = new.product_line and case
when new.product_variant is null then product_variant is not null
when new.product_variant is not null then product_variant is null
end and
area_nr = new.area_nr and
net = new.net and
period = new.period
) then
raise exception 'collision of distributer % with related sub-distributers for area % and net % in period %', new.product_line, new.area_nr, new.net, new.period;
else
return new;
end if;
end $$ language plpgsql;
create trigger before_insert_incomplete_distributer before insert on booking for each row
execute procedure _avoid_incomplete_distributers();

Answer 1

You can do this without triggers using write your own check function and adding to table check constraint.
Fox example:
This is function sample:
CREATE OR REPLACE FUNCTION check_booking(v_pline text, v_pvar text, v_area int2)
RETURNS bool
LANGUAGE plpgsql
AS $function$
BEGIN
if (v_pvar is null) then
if (exists(select 1 from booking where product_line = v_pline and area_nr = v_area)) then
return false;
end if;
end if;
if (v_pvar is not null) then
if (exists(select 1 from booking where product_line = v_pline and area_nr = v_area and product_variant is null)) then
return false;
end if;
end if;
return true;
END;
$function$
This is add constraint sample:
ALTER TABLE booking
ADD CONSTRAINT booking_check_constraint
CHECK (check_booking(product_line, product_variant, area_nr));