Question

I’m working with mySQL, and I’m in a situation where I need to select data from one table that matches an ID at any level in parent -> child data hierarchy in the other table.

Further more, I would like to resolve this with a well written SQL query, rather than a recursive function in my PHP code, as this feature will be used quite a bit.

I did try searching, and I have stumbled upon numerous similar problems (most of them being resolved), however none of them helped me.

To help illustrate the situation, here’s my current setup

table “articles”:

• article_id
• category_id
• …

table categories

• category_id
• parent_id
• …

I need to select all the articles from “articles” where “articles.category_id” is, let’s say, 10. But also receive all the articles from all categories from the tree the “categories.category_id” 10 belongs to.

Meaning, where “10” is the parent and all of it’s children, and upwards where 10 is the child and all of it’s parents.

Possible without a recursive php function?

Thank you.

Answer 1

This is possible to do in MySQL, but it takes a little effort. You’ll have to write a function like this:

CREATE FUNCTION hierarchy_connect_by_parent_eq_prior_id(value INT) RETURNS INT
NOT DETERMINISTIC
READS SQL DATA
BEGIN
        DECLARE _id INT;
        DECLARE _parent INT;
        DECLARE _next INT;
        DECLARE CONTINUE HANDLER FOR NOT FOUND SET @id = NULL;

        SET _parent = @id;
        SET _id = -1;

        IF @id IS NULL THEN
                RETURN NULL;
        END IF;

        LOOP
                SELECT  MIN(id)
                INTO    @id
                FROM    categories
                WHERE   parent = _parent
                        AND id > _id;
                IF @id IS NOT NULL OR _parent = @start_with THEN
                        SET @level = @level + 1;
                        RETURN @id;
                END IF;
                SET @level := @level - 1;
                SELECT  id, parent
                INTO    _id, _parent
                FROM    categories
                WHERE   id = _parent;
        END LOOP;
END

and use it in a query:

SELECT  id, parent, level
FROM    (
        SELECT  hierarchy_connect_by_parent_eq_prior_id(id) AS id, @level AS level
        FROM    (
                SELECT  @start_with := 0,
                        @id := @start_with,
                        @level := 0
                ) vars, categories 
        WHERE   @id IS NOT NULL
        ) ho
JOIN    categories hi
ON      hi.id = ho.id

See this entry in my blog for more detail:

• Hierarchical queries in MySQL