mySQL query for selecting children

Question

I am not sure if this is possible in mySQL. Here are my tables:-

Categories table:

• id
• name
• parent_id (which points to Categories.id)<

Answer 1

Add a column to the Categories table that will contain the complete comma-delimited tree for each group. Using your example, sub-category Educational would have this as the tree '1,2', where 1 = Toys, 2 = Educational (it includes itself). The next nested level of categories would keep adding to the tree.

To get all products in a group, you use MySQL's FIND_IN_SET function, like so

SELECT p.ID 
FROM Products p INNER JOIN Categories c ON p.category_ID = c.ID
WHERE FIND_IN_SET(your_category_id, c.tree)

I wouldn't use this method for big tables, as I don't think this query can use an index.