如何从这两个表计算利润或损失？

create table items(id int primary key auto_increment, name varchar(255)); insert into items value(null,'A'); insert into items value(null,'B'); insert into items value(null,'C'); insert into items value(null,'D');

create table purchase(id int primary key auto_increment, item_id int,qnt int,price int); insert into purchase value(null,1,10,20); insert into purchase value(null,2,10,22); insert into purchase value(null,3,10,25); insert into purchase value(null,4,10,18); insert into purchase value(null,5,10,25);

create table sales(id int primary key auto_increment, item_id int,qnt int,price int); insert into purchase value(null,1,2,25); insert into purchase value(null,2,3,15); insert into purchase value(null,1,3,26); insert into purchase value(null,1,2,22);

我需要的是将采购表中每件商品的成本像这样，(salespriceqnt)-( costpriceqnt) 作为利润/损失

不知道该怎么做，现在我正在使用 PHP 来做这件事，这需要太多时间。