I need some quality sources - books, websites etc. - to educate myself about putting multiple hierarchy trees in a single table using SQL.
I'm looking for some good theoretical and practical information.
You can put multiple hierarchies in an Oracle table and query it using a hierarchical query. See the documentation.
What we have done with LedgerSMB for our business reporting dimensions (each dimension is hierarchical) is to have a dimensions table, and a units table. Each member of the units belongs to a dimension, and we define hierarchy using the self join of (parent, unit_class) references (id, unit_class). This way we have multiple equivalent hierarchies sharing a table.