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> > hierarchies fail, and this is my struggle with xml at the
> > moment, when they have to support multiple hierarchies simultaneously.
>
> Perhaps it's time to rediscover the network (or Codasyl) data model, which
> was invented to solve this problem.
The network model from the CODASYL DBTG supports many-to-many relationships but,
like a hierarchical database, it's based on pointers. Childs' STDS and Codd's
relational model emerged as alternatives -- being able to retrieve by value and
not by physical position.
There are several SQL solutions for processing hierarchies or trees, including
proprietary SQL extensions, adjacency lists, nested sets, and materialized
paths. Some of the techniques (e.g., preorder tree traversal) were/are used with
hierarchical DBMS products such as IBM IMS.
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