Anyways I have to store a hierarchical data structure, similar to a tree in a SQL database.<BR><BR>Right now I simply add a "parent_id" column in each of the row (node).

A database organization method that is structured in a hierarchy. All access to data starts at the top of the hierarchy and moves downward; for example, from customer to orders, vendor to ...

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You can create meaningful reports from data in a database. Your data has a built-in structure to it. Information of a given type is always stored only once. Databases are ACID.