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Thread: Aggregate functions on MPTT / Nested Set Model.

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  1. #1
    Join Date
    Mar 2011
    Posts
    2

    Aggregate functions on MPTT / Nested Set Model.

    So I'm using MySQL and storing my category structure based on the Nested Set / Modified Pre-Order Tree Traversal model, in a table named "nested_category" which has fields: category_id, name, lft, rgt, published

    published is either 1 or 0... If it's 1 then the category will show upon the live site. If it's 0 then it won't show on the live site, and more importantly any children of that unpublished category won't show on the live site either.

    I'm having problems writing a query to list all the categories that have published=1, and ignore all categories that are descendants of a category which has published=0.

    At the moment I'm using:

    Code:
    SELECT category_id, lft, rgt FROM nested_category WHERE published = 1
    I really can't work out how to get it to ignore "child" categories when a parent is "unpublished".

    I'm also trying to link this to my "new_products" table which has fields: product_id, name, stock, price, category_id, so that I can write a query to select all products that have published=1 and are part of a "published" category. I have got this far:

    Code:
    select @myRight := rgt, @myLeft := lft 
    from nested_category where name="ELECTRONICS";
    
    select productId, productName, new_products.category_id, 
    price, stock, new_products.published 
    from new_products 
    inner join ( 
        select category_id, lft, rgt from nested_category 
        where published = 1
    ) cat 
    on new_products.category_id = cat.category_id 
    and cat.lft >= @myLeft 
    and cat.rgt <= @myRight 
    and new_products.published = 1 
    order by productName asc
    Since the above query uses my first query, it doesn't return any "unpublished" categories or products, but it doesn't take into account when a "published" category is a descendant of an "unpublished" category. Hope this makes sense!

  2. #2
    Join Date
    Mar 2011
    Posts
    2
    Update: after playing around a lot I've hacked together a query that works, but isn't as efficient as I know it could be.

    Code:
    SELECT nested_category.name, nested_category.category_id, nested_category.lft, nested_category.rgt 
    FROM nested_category 
    LEFT JOIN (
        SELECT nested_category.category_id, nested_category.lft, nested_category.rgt 
        FROM nested_category, (
            SELECT category_id, lft, rgt 
            FROM nested_category 
            WHERE published = 0
        ) notCat 
        WHERE nested_category.lft >= notCat.lft 
        AND nested_category.rgt <= notCat.rgt ) notCat2
    ON notCat2.category_id=nested_category.category_id 
    WHERE notCat2.category_id IS NULL
    ORDER BY nested_category.lft ASC
    This returns all the categories that are online, and ignores categories that are online but are "blocked" by a parent being "unpublished" somewhere up the tree.

    I'm still a bit hazy on the JOINs so if someone could help me re-write it using JOINs instead of sub-queries I know it'll be more efficient!

    I hope this helps someone in future with the MPTT, it's so confusing sometimes!

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