Postgresql data validation checks based on table values - postgresql

Question

I'm trying to create a database that tracks electrical cables. Each cable contains 1 or more cores that are connected to terminals at each end. The number of cores in each cable is defined in a table.
| number_of_cores | cable_id
|----------2-------|---1-----|
The core table is as follows
cable_no | from_id | core_mark | to_id
1001 | 1 | 1 Black | 2
1001 | 2 | 1 White | 4
I want to create a check that will prevent another 1001 cable core from being inserted.
Is this possible in postgresql?
Ideally, if I tried to insert another 1001 cable with another unique core number, the error would be something like "all cores used on cable 1001"
Thanks,

Answer 1

I think what you need is something like a check constraint. (https://www.postgresql.org/docs/current/ddl-constraints.html)
Follow those steps :
1. Create some table properly
create table cable (cable_id int primary key, number_of_cores int);
create table core (core_id int primary key, cable_id int references cable (cable_id), from_id int, core_mark varchar (50), to_id int);
2. Create the function that will verify the inserts
create or replace function test_max_core_number(in_cable_id int)
returns boolean
language plpgsql
as $function$
declare
res boolean := false;
begin
if exists (
select *
from cable
where cable_id = in_cable_id
and number_of_cores > (select count(*) from core where cable_id = in_cable_id )
)
then
res := true;
end if;
return res;
end;
$function$;
3. Add the constraint to your table
alter table core
add constraint cstr_check check (test_max_core_number(cable_id));
4. Now it is time for some testing :)
insert into cable (cable_id, number_of_cores) values (1, 2), (2, 3);
insert into core (core_id, cable_id, from_id, core_mark, to_id)
values
(1, 1, 1, '1 Black', 2)
,(2, 1, 2, '1 White', 4);
Normally all goes fine for now.
5. And now the wanted error !
insert into core (core_id, cable_id, from_id, core_mark, to_id)
values
(3, 1, 3, '1 Green', 2);
Hope this helps !

Answer 2

I think #Jaisus gave a good answer.
I would add only a cross-check into the cable to prevent to set bad values into number_of_cores:
create or replace function test_cable_number_of_cores(in_cable_id int,in_number_of_cores int)
returns boolean
language plpgsql
as $function$
declare
res boolean := false;
begin
res := (in_number_of_cores>0 and (select count(cable_id) from core where cable_id=in_cable_id) <= in_number_of_cores);
return res;
end;
$function$;
alter table cable add check(test_cable_number_of_cores(cable_id, number_of_cores));
-- ok
insert into cable(cable_id, number_of_cores) values (3, 2);
update cable set number_of_cores=3 where cable_id=3;
-- error
update cable set number_of_cores=1 where cable_id=1;