Question

products(id, name)
categories(id, name, product_id)

I have a sample code:

products(1, 'Apple')

And categories

categories(1, 'Iphone', 1)
categories(2, 'Ipad', 1)
categories(3, 'Ipod', 1)

And query:

SELECT pr.id, cat.name, pr.name 
FROM `products` as pr
LEFT JOIN `categories` as cat USING(id)
GROUP BY pr.id

But result is

1, "Iphone", "Apple"

And Ipad, Ipod not show, how to fix it?

 1, "Iphone, Ipad, Ipod", "Apple"

Answer 1

Your JOIN condition is attempting to join the two ID columns, which is not the relationship. It should join the product.id to the category.produt_id

SELECT pr.id, cat.name, pr.name 
FROM `products` as pr
LEFT JOIN `categories` as cat ON pr.id = cat.product_id
GROUP BY pr.id

EDIT:
I realize that you want only one result with the categories concatenated. You need the GROUP_CONCAT function in your select list:

SELECT pr.id, GROUP_CONCAT(cat.name), ...

http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_group-concat