How to handle a attribute with just two possible values - sql

Question

In my web application i have a company which can be a buyer or a supplier or both.
So my database tables would be like this
Company( id_company, ..., is_buyer, is_supplier, ... )
Or :
Company( id_company, ... )
Type_company( id_type_company, type )
Extra_table(id_company, id_type_company )
Or :
Company( id_company, ... )
Type_company( id_company, id_type_company, type )
I want a explication (Pros and Cons) for every case if it's possible.

Answer 1

You can consider using the common supertype like this
CREATE TABLE companies
(
id int not null primary key,
name varchar(128)
-- other columns
);
CREATE TABLE buyers
(
company_id int not null primary key,
foreign key (company_id) references companies (id)
);
CREATE TABLE suppliers
(
company_id int not null primary key,
foreign key (company_id) references companies (id)
);
Here are some sample queries:
-- Select all buyers
SELECT c.id, c.name
FROM companies c JOIN buyers b
ON c.id = b.company_id;
-- Select all suppliers
SELECT c.id, c.name
FROM companies c JOIN suppliers s
ON c.id = s.company_id;
-- Select companies that are both buers and suppliers
SELECT c.id, c.name
FROM companies c JOIN buyers b
ON c.id = b.company_id JOIN suppliers s
ON c.id = s.company_id;
-- Select companies that are buers BUT NOT suppliers
SELECT c.id, c.name
FROM companies c JOIN buyers b
ON c.id = b.company_id LEFT JOIN suppliers s
ON c.id = s.company_id
WHERE s.company_id IS NULL;
Here is SQLFiddle demo
Recommended reading:
SQL Antipatterns by #BillKarwin