MySQL: how to query parent-child?

Question

Assume the following table records:

TABLE: foo
==========================
| foo_id | foo_parent_id |
==========================
| 1      | NULL          |
|          


        

Answer 1

I'd like to offer another approach at answering this question:

SET @topN = 1;
SELECT foo_id, foo_parent_id
FROM (
    SELECT 
          f.foo_id
        , f.foo_parent_id
        , IFNULL(f.foo_parent_id, f.foo_id) AS grp_rank
        , CASE 
            WHEN f.foo_parent_id IS NULL 
                THEN @topN:= 1 
            ELSE 
                @topN:=@topN+1 
            END topN_rank
        , f_parent.foo_id AS f_parent_foo_id
    FROM 
        foo AS f
        RIGHT JOIN (
            SELECT foo_id 
            FROM foo 
            WHERE foo_parent_id IS NULL 
            ORDER BY foo_id LIMIT 10
        ) f_parent
        ON f_parent.foo_id = IFNULL(f.foo_parent_id, f.foo_id)
    ORDER BY grp_rank, f.foo_id
) AS foo_derived
WHERE topN_rank <= 3

General Notes:

I want to get, say, the first 10 parent records <-- SUBQUERY "f_parent"

followed by, say, the first 2 child records of that parent record. <-- topN <= 3 (PARENT + 2 CHILDREN)

How it works:

1. Create an extra column for foo_id to foo_parent_id integration grp_rank
2. Join to a subquery pulling only the top 10 parents (assuming the order is by their foo_id values)
3. Remember to sort by that extra column followed by foo_id for 'parent..followed by..first two children'
4. Create another column than ranks the rows that will be sorted resetting via foo_parent encounters. NULL foo_parent_id resets the ranking. This new column topN_rank is impacted by the sorting.
5. Nest that query to be able to derive topN_rank and to return only the needed columns.
6. Filter by topN_rank column to get the top N rows for each group, prohibiting more than 3 per group.