ON DELETE SET NULL
is a standard foreign key constraint option.
CREATE TABLE some_child (
parent_id integer references parent(id) on delete set null
);
or:
ALTER TABLE some_child
ADD CONSTRAINT parent_id_fk
FOREIGN KEY (parent_id) REFERENCES parent(id)
ON DELETE SET NULL;
See the documentation.
In future posts make sure you include your PostgreSQL version and explain what you’ve already tried.