Question

I have a table product_category, with fields id, name, parent_id and level.

For example, the category Agriculture has id = 75, level = 1 and parent_id = NULL. The subcategories Corn, Wheat, Rye, etc. have level = 2 and parent_id = 75.

In my site I’d like to show the top-level categories and below each of them, only 5 subcategories. But doing a query to retrieve them it’s harder than what I thought.

If I do the following query:

SELECT a.name, a.parent_id FROM product_category a
WHERE (
    SELECT b.level
    FROM product_category b
    WHERE b.id = a.parent_id
    LIMIT 1
) = 1

I retrieve all top-level categories and subcategories, but there are thousands of subcategories so it would be very unnecessarily expensive when I only want the first 5 of each.

If I do the following one:

SELECT a.name, a.parent_id FROM product_category a
WHERE (
    SELECT b.level
    FROM product_category b
    WHERE b.id = a.parent_id
    LIMIT 1
) = 1
LIMIT 5

It only retrieves 5 subcategories, not 5 subcategories per top-level category.

Then I thought of doing it the following way:

(
     SELECT a.name, a.parent_id FROM product_category a
     WHERE parent_id = 12
     LIMIT 5
) UNION (
     SELECT a.name, a.parent_id FROM product_category a
     WHERE parent_id = 21
     LIMIT 5
) UNION (     
     SELECT a.name, a.parent_id FROM product_category a
     WHERE parent_id = 75
     LIMIT 5
) UNION (
.
.
.

Which looks very dirty and hard-coded, yet it is the only way I can think of right now. Is there any other solution to this?

Thanks!

Answer 1

Here’s an example to return up to two subcategories per root category:

select  parent.name as Category
,       child.name as SubCategory
from    (
        select  name
        ,       parent_id
        ,       @rn := if(@cur = parent_id, @rn+1, 1) as rn
        ,       @cur := parent_id
        from    product_category pc
        join    (select @rn := 0, @cur := '') i
        where   level = 2
        order by
                parent_id
        ,       id
        ) as child
join    product_category as parent
on      child.parent_id = parent.id
where   child.rn < 3

Live example at SQL Fiddle.