Question

Duplicate:

SQL – how to store and navigate hierarchies

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If I have a database where the client requires categories, sub-categories, sub-sub-categories and so on, what’s the best way to do that? If they only needed three, and always knew they’d need three I could just create three tables cat, subcat, subsubcat, or the like. But what if they want further depth? I don’t like the three tables but it’s the only way I know how to do it.

I have seen the "sql adjacency list" but didn’t know if that was the only way possible. I was hoping for input so that the client can have any level of categories and subcategories. I believe this means hierarchical data.

EDIT: Was hoping for the sql to get the list back out if possible

Thank you.

Answer 1

table categories: id, title, parent_category_id


 id | title | parent_category_id
----+-------+-------------------
  1 |  food |              NULL
  2 | pizza |                 1
  3 | wines |              NULL
  4 |   red |                 3
  5 | white |                 3
  6 | bread |                 1

I usually do a select * and assemble the tree algorithmically in the application layer.