postgres - avoid creating duplicate null columns-CodePudding

I have this table schema on Postgres:

> \d  users_types_brands

                   Table "public.users_types_brands"
     Column     |            Type             | Collation | Nullable |                    Default                     | Storage | Stats target | Description 
---------------- ----------------------------- ----------- ---------- ------------------------------------------------ --------- -------------- -------------
 id             | integer                     |           | not null | nextval('users_types_brands_id_seq'::regclass) | plain   |              | 
 inserted_at    | timestamp without time zone |           |          | now()                                          | plain   |              | 
 updated_at     | timestamp without time zone |           |          | now()                                          | plain   |              | 
 users_types_id | bigint                      |           |          |                                                | plain   |              | 
 brand_id       | bigint                      |           | not null |                                                | plain   |              | 
 tasks_type_id  | integer                     |           |          |                                                | plain   |              | 
Indexes:
    "users_types_brands_pkey" PRIMARY KEY, btree (id)
    "users_types_brands_users_types_id_brand_id_tasks_type_id_index" UNIQUE, btree (users_types_id, brand_id, tasks_type_id)
Foreign-key constraints:
    "users_types_brands_users_types_id_fkey" FOREIGN KEY (users_types_id) REFERENCES users_types(id)
Access method: heap

Right now the table looks like this:

my_db=# select * from users_types_brands;
 id |        inserted_at         |         updated_at         | users_types_id | brand_id | tasks_type_id 
---- ---------------------------- ---------------------------- ---------------- ---------- ---------------
 12 | 2021-10-24 16:43:12.244026 | 2021-10-24 16:43:12.244026 |              2 |      112 |             8
 14 | 2021-10-24 17:03:12.012874 | 2021-10-24 17:03:12.012874 |              2 |      111 |             9
(2 rows)

Of course, I can't insert a row like this:

my_db=# insert into users_types_brands (users_types_id, brand_id, tasks_type_id) values (2, 112, 8);
ERROR:  duplicate key value violates unique constraint "users_types_brands_users_types_id_brand_id_tasks_type_id_index"
DETAIL:  Key (users_types_id, brand_id, tasks_type_id)=(2, 112, 8) already exists.

But I can do this several times:

my_db=# insert into users_types_brands (users_types_id, brand_id) values (2, 112);
INSERT 0 1

And obtain this:

my_db=# select * from users_types_brands;
 id |        inserted_at         |         updated_at         | users_types_id | brand_id | tasks_type_id 
---- ---------------------------- ---------------------------- ---------------- ---------- ---------------
 12 | 2021-10-24 16:43:12.244026 | 2021-10-24 16:43:12.244026 |              2 |      112 |             8
 14 | 2021-10-24 17:03:12.012874 | 2021-10-24 17:03:12.012874 |              2 |      111 |             9
 16 | 2021-10-24 17:15:58.295428 | 2021-10-24 17:15:58.295428 |              2 |      112 |              
 17 | 2021-10-24 17:16:36.99971  | 2021-10-24 17:16:36.99971  |              2 |      112 |              
(4 rows)

Now, according to the business rules, tasks_type_id can be null