How to set up a foreign key for ON DELETE SET NULL, ie when a parent row is deleted, matching key values in child rows are set to NULL:
drop table if exists b,a;
create table a(id int primary key)engine=innodb;
insert into a values(1),(2);
create table b(
id int primary key,
aid int,
key(aid),
foreign key(aid) references a(id) on delete set null
)engine=innodb;
insert into b values(1,1),(2,1);
delete from a where id=1;
select * from b;
+----+------+
| id | aid |
+----+------+
| 1 | NULL |
| 2 | NULL |
+----+------+
|
|