正如我在评论中所说，您可以使用window-functions-usage来完成
架构代码：

create table mkt_product(
  product_id varchar(100),
 category_id varchar(100),
 product varchar(100)
);
insert into mkt_product(product_id,category_id,product) 
values
('001','cat1','Washing machine'),
('002','cat1','Refrigerator'),
('003','cat1','Oven'),
('004','cat1','Built in Oven'),
('005','cat1','Microwave'),
('006','cat1','Vacuum cleaner'),
('007','cat1','Hob'),
('008','cat2','Potato'),
('009','cat2','Carrot'),
('010','cat2','Onion'),
('011','cat2','Garlic'),
('012','cat2','Tomato'),
('013','cat2','Cucumber'),
('014','cat2','Lettuce');

查询sql:

SELECT t1.*
  FROM mkt_product t1
  JOIN
  (
     SELECT product_id,category_id,product,
     ROW_NUMBER() OVER(PARTITION BY category_id ORDER BY product_id) as rn  
     FROM mkt_product
   ) t2 ON t1.product_id = t2.product_id 
     AND t1.category_id=t2.category_id AND t1.product=t2.product
  WHERE rn<=5

注意：t1.product_id = t2.product_id AND t1.category_id=t2.category_id AND t1.product=t2.product非常难看，如果你的表有id这样的主键，它会更简单
DB Fiddle Demo