Select products where the category belongs to any category in the hierarchy

Question

I have a products table that contains a FK for a category, the Categories table is created in a way that each category can have a parent category, example:

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Answer 1

The best solution for this is at the database design stage. Your categories table needs to be a Nested Set. The article Managing Hierarchical Data in MySQL is not that MySQL specific (despite the title), and gives a great overview of the different methods of storing a hierarchy in a database table.

Executive Summary:

Nested Sets

• Selects are easy for any depth
• Inserts and deletes are hard

Standard parent_id based hierarchy

• Selects are based on inner joins (so get hairy fast)
• Inserts and deletes are easy

So based on your example, if your hierarchy table was a nested set your query would look something like this:

SELECT * FROM products 
   INNER JOIN categories ON categories.id = products.category_id 
WHERE categories.lft > 2 and categories.rgt < 11

the 2 and 11 are the left and right respectively of the Processors record.