How can I add a constraint to an Oracle table to make sure, each row in the table is referenced at least once in another table?

In my oracle DB i have table A that is referenced through its ID column in table B. Though, currently, not every row of A is referenced in B. I would like to add a contraint of sorts or a virtual column to A that ensures that for EVERY row in A there is at least one row in table B or, in other words, every table A row is referenced by at least one row in table B. But i don't know how to go about it. i thought about doing a deferrable virtual column in table A like in this pseudo code here:

alter table A add (b_id as (
    case select id into b_id from the first row of B that references A
        END) unique deferrable initially deferred);

but this doesn't make sense to me and, i'm not even sure if it is at all possible to reference another table in the virtual column of a table. i feel like, as a newbie, there might be some better options out there. how would you go about it, any ideas?

If you want a parent-child relational where every parent has (at least) one child then you can create a deferrable foreign key from the parent back to the child:

create table employees ( 
employee_id   int primary key,
first_name    varchar2(100) not null, 
last_name     varchar2(100) not null, 
department_id int not null
);
create table departments ( 
department_id   int primary key, 
department_name varchar2(30) not null, 
manager_id      int not null
);
alter table employees
add constraint empl_department_fk
foreign key ( department_id )
references departments;
alter table departments
add constraint dept_manager_fk
foreign key ( manager_id )
references employees ( employee_id )
deferrable;
/* Delay deferrable constraint validation */
alter session set constraints = deferred;
insert into departments ( department_id, department_name, manager_id )
values ( 42, 'Test dept', 42 );
/* Constraint is validated here - raise error & rollback */
commit;
--ORA-02291: integrity constraint (CHRIS.DEPT_MANAGER_FK) violated - parent key not found
insert into departments ( department_id, department_name, manager_id )
values ( 42, 'Test dept', 42 );
insert into employees ( employee_id, first_name, last_name, department_id )
values ( 42, 'Tess', 'Ting', 42 );
commit;

You can use this whenever each parent row has a default/primary/owner row from the child rows. This is more common than often assumed - e.g. Every department must have a manager, every customer must have a default payment method, every person must have a primary contact method.

If there is no default/primary/owner, you can enforce this by creating a fast refresh on commit materialized view with a query outer joining the child to the parent. Add a (deferrable) not null constraint over child columns in the MV and voila:

create materialized view log on employees
with rowid, primary key ( department_id ),
sequence
including new values;
create materialized view log on departments
with rowid, primary key, 
sequence
including new values;
/* 
Is there a dept with no employees?
MV outer joining child to parent 
*/
create materialized view department_employees_mv
refresh fast on commit
as
select e.rowid empl_rid, d.rowid dept_rid,
e.employee_id, d.department_id
from   employees e, departments d
where  d.department_id = e.department_id (+);
/* Check child columns are not null */
alter table department_employees_mv 
modify employee_id 
constraint deem_employee_nn
not null
deferrable;
/* Prove MV works - drop dept -> emp FK */
alter table departments
drop constraint dept_manager_fk;
/* Add new dept with no employees */
insert into departments ( department_id, department_name, manager_id )
values ( 99, 'test', 99 );
/* MV query - note employee_id is null */
select e.employee_id, d.department_id
from   employees e, departments d
where  d.department_id = e.department_id (+)
and    d.department_id = 99;
EMPLOYEE_ID DEPARTMENT_ID
----------- -------------
<null>                 99
commit;
--ORA-02091: transaction rolled back
--ORA-02290: check constraint (CHRIS.DEEM_EMPLOYEE_NN) violated
/* Insert rolled back */
select e.employee_id, d.department_id
from   employees e, departments d
where  d.department_id = e.department_id (+)
and    d.department_id = 99;
insert into departments ( department_id, department_name, manager_id )
values ( 99, 'test', 99 );
insert into employees ( employee_id, first_name, last_name, department_id )
values ( 99, 'Tess', 'Ting', 99 );
commit;
select * from department_employees_mv
where  department_id = 99;
EMPL_RID           DEPT_RID           EMPLOYEE_ID DEPARTMENT_ID
------------------ ------------------ ----------- -------------
AAAToCAAPAAAANHAAB AAAToEAAPAAAAMPAAB          99            99

For more on this see my recent Ask TOM Office Hours on Advanced Database Constraints.