How to add delete cascade on staging table - sql

Question

Here how I created tables:
CREATE TABLE TABLE_A(
id uuid NOT NULL, UNIQUE
name text
);
CREATE TABLE TABLE_B(
id uuid NOT NULL, UNIQUE
name text
);
-- custom realization of many-to-many association
CREATE TABLE TABLE_A_B(
id uuid NOT NULL, UNIQUE
a_id uuid REFERENCES TABLE_A(id) ON UPDATE CASCADE,
B_id uuid REFERENCES TABLE_B(id) ON UPDATE CASCADE
);
I've already created tables and now can't update it by adding ON DELETE CASCADE.
And I need now to add ON DELETE CASCADE to staging table TABLE_A_B. How to do it ?(

Answer 1

You use ON DELETE CASCADE:
CREATE TABLE TABLE_A_B(
id uuid NOT NULL UNIQUE,
a_id uuid REFERENCES TABLE_A(id) ON UPDATE CASCADE ON DELETE CASCADE,
B_id uuid REFERENCES TABLE_B(id) ON UPDATE CASCADE ON DELETE CASCADE
);
Here is a db<>fiddle that fixed some typos in your code.
In particular, the foreign key reference should be to a primary key. Although allowed to a unique key, the purpose of primary keys is really to identify individual rows -- and one main use is for foreign key references.
EDIT:
If the constraints already exist, then do the following.
First, get their names:
select *
from information_schema.table_constraints
where constraint_type = 'FOREIGN KEY' and table_name = 'table_a_b';
Note: You can assign names to skip this step.
Then drop the existing foreign key constraint:
alter table table_a_b
drop constraint table_a_b_a_id_fkey;
Finally, add a new one:
alter table table_a_b
add constraint fk_table_a_b_a
foreign key (a_id) references table_a(id)
on update cascade
on delete cascade;