SELECT p.id, d.name, p.item, p.price, min(p.price) over(), p.price-min(p.price) over() FROM purchase p join department d on p.department_id=d.id
SELECT p.id, d.name, p.item, p.price, min(p.price) over(), p.price-min(p.price) over() FROM purchase p join department d on p.department_id=d.id