Deleting related rows from other tables without primary key - sql

Question

I need to delete the associated rows from the "assign" table when a crew member is deleted.
However, when I create the tables, I get the error:
ERROR: there is no unique constraint matching given keys for referenced table "assign"
It seems as though I need distinct values in the "assign" table, but the whole point is that I want to delete all associated information when a crew member is removed. How can I do this?
create table flight(
flight_num BIGSERIAL PRIMARY KEY,
source_city varchar,
dest_city varchar,
dep_time int,
arr_time int,
airfare int,
mileage int
);
create table crew (
id BIGSERIAL PRIMARY KEY,
name varchar,
salary int,
position varchar,
seniority int,
fly_hours int,
mgrid int,
FOREIGN KEY (id) REFERENCES assign(id) ON DELETE CASCADE
);
create table assign (
id int, # refers to a crew member id, not a row id
flight_num int
);

Answer 1

The foreign keys should be on the M-M join table, as so:
create table flight(
flight_num int PRIMARY KEY,
-- etc
);
create table crew (
crew_id int PRIMARY KEY,
-- etc
);
create table assign (
crew_id int,
flight_num int,
-- If the corresponding record in either of these FKs is deleted then
-- this record will be deleted.
FOREIGN KEY (crew_id) REFERENCES crew(crew_id) ON DELETE CASCADE,
FOREIGN KEY (flight_num) REFERENCES flight(flight_num) ON DELETE CASCADE,
-- M-M usually should have a PK over both foreign columns, arranged in order
-- of expected usage. A reverse covering index can be added if needed.
PRIMARY KEY (flight_num, crew_id)
);
In this case the FKs are allowed because they are over a unique columns (PKs of crew and flight). The original error is because there was no unique contraint on the FK target (assign.crew_id).