Indirect UNIQUE constraint - sql

Question

I've had a following set of tables:
CREATE TABLE sellers (
id integer PRIMARY KEY,
....
);
CREATE TABLE buyers (
id integer PRIMARY KEY,
....
);
CREATE TABLE invoices (
id integer PRIMARY KEY,
number varchar(40),
buyer_id integer REFERENCES buyers(id),
seller_id integer REFERENCES sellers(id),
UNIQUE(seller_id, number),
....
);
The UNIQUE constraint is enforces the constraint that one seller can have only one invoice with given number.
I wanted to create a new table, mostly as a many-to-many relationship between buyers and sellers:
CREATE TABLE suppliers (
id integer PRIMARY KEY,
buyer_id integer REFERENCES buyers(id),
seller_id integer REFERENCES sellers(id),
....
);
As a normalization effort, i would like to change invoices to refer to suppliers, like this:
CREATE TABLE invoices (
id integer PRIMARY KEY,
number varchar(40),
supplier_id integer REFERENCES suppliers(id),
....
);
My question is: how can I replace the UNIQUE constraint?

Answer 1

Use a composite primary key for suppliers:
CREATE TABLE suppliers (
buyer_id integer REFERENCES buyers(id),
seller_id integer REFERENCES sellers(id),
...
PRIMARY KEY (buyer_id, seller_id),
...
);
and keep your invoices table intact, only changing the two foreign keys into one:
CREATE TABLE invoices (
id integer PRIMARY KEY,
number varchar(40),
buyer_id integer,
seller_id integer,
UNIQUE (seller_id, number),
FOREIGN KEY (buyer_id, seller_id)
REFERENCES suppliers (buyer_id, seller_id),
....
);
I guess that this design will be useful, if you plan to have more columns in the suppliers table that are related to the seller - buyer relationship. Otherwise, you could have a view that gathers data from the invoices table:
CREATE VIEW suppliers AS
SELECT DISTINCT buyer_id, seller_id
FROM invoices ;