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

What you want to find is the transitive closure of the category "parent" relation. I suppose there's no limitation to the category hierarchy depth, so you can't formulate a single SQL query which finds all categories. What I would do (in pseudocode) is this:

categoriesSet = empty set
while new.size > 0:
  new = select * from categories where parent in categoriesSet
  categoriesSet = categoriesSet+new

So just keep on querying for children until no more are found. This behaves well in terms of speed unless you have a degenerated hierarchy (say, 1000 categories, each a child of another), or a large number of total categories. In the second case, you could always work with temporary tables to keep data transfer between your app and the database small.