Postgres 12

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table p
( a int not null
, b int
, unique (a,b)
);

CREATE TABLE

create table c
( a int not null
, b int
, constraint fk_p foreign key (a,b)
                  references p (a,b)
                      on delete cascade
                      on update cascade
);

CREATE TABLE

insert into p (a,b) values (1,null);

INSERT 0 1

insert into c (a,b) values (1,null), (2,null);

INSERT 0 2

delete from p;

DELETE 1

select * from c;

a
b

1
null

2
null

SELECT 2

a	b
1	null
2	null