postgresql create table many-to-many
CREATE TABLE product (
product_id serial PRIMARY KEY -- implicit primary key constraint
, product text NOT NULL
, price numeric NOT NULL DEFAULT 0
);
CREATE TABLE bill (
bill_id serial PRIMARY KEY
, bill text NOT NULL
, billdate date NOT NULL DEFAULT CURRENT_DATE
);
CREATE TABLE bill_product (
bill_id int REFERENCES bill (bill_id) ON UPDATE CASCADE ON DELETE CASCADE
, product_id int REFERENCES product (product_id) ON UPDATE CASCADE
, amount numeric NOT NULL DEFAULT 1
, CONSTRAINT bill_product_pkey PRIMARY KEY (bill_id, product_id) -- explicit pk
);