Find the names of companies that produce DVD drives.
select t2.name from products as t1 join manufacturers as t2 on t1.manufacturer = t2.code where t1.name = 'DVD drive'
how about the companies that do not make them?
select name from manufacturers except select t2.name from products as t1 join manufacturers as t2 on t1.manufacturer = t2.code where t1.name = 'DVD drive'