复杂查询-MySql（类别x产品）

Question

I have difficult about a complex query. There is a table for categories and for products. I need show 4 products of each category. How can I do that?

My struct: Category.id, Category.name and Product.id, Product.category_id.

SQL:

SELECT Product.id, Product.name, Product.img, Category.name FROM categories AS Category
INNER JOIN (SELECT Product.id, Product.name, Product.img, Product.category_id FROM products AS Product) AS Product ON
Product.category_id = Category.id;

Where do I put the limits for products?

Thanks.

Felipe Marques

Answer 1

This is painful in MySQL (in my opinion). Here is a version with a correlated subquery:

SELECT p.id, p.name, p.img, c.name
FROM categories c INNER JOIN
     (SELECT p.id, p.name, p.img, p.category_id,
             (select count(*) from products p2 where p2.category_id = p.category_id and p2.id <= p.id
             ) as seqnum
      FROM products p
     ) p
     ON Product.category_id = Category.id
where seqnum <= 4

This uses the correlated subquery to count the number of products who product id is less than or equal to the given product. This creates a sequence number, where you can select the first four.