MySQL 触发器。需要示例答案

Question

我正在尝试寻找只生产笔记本电脑而不生产 PC 的产品制造商

Relations:
Product(maker, model, type)
Laptop(Model, price, speed, ram, hd, screen)
PC(model, price, speed, ram, hd)
Printer(model,price, color, price)

我尝试过的

 (SELECT maker, type 
 FROM product WHERE type = 'laptop') 
 DIFFERENCE 
 (SELECT maker, type 
 FROM product WHERE type = 'pc');

我想在MySQL中运行没有区别吧？

Answer 1

SELECT 
    p1.maker, 
    p1.type
FROM 
    product p1 
    LEFT JOIN product p2 ON p2.maker = p1.maker AND p2.type = 'pc'
WHERE 
    p1.type = 'laptop'
    p2.maker IS NULL