Much of the data we deal with every day is organized hierarchically: file s
ystems, library classification schemes and yellow page categories are salie
nt examples. Business data too, benefits from a hierarchical organization,
and indeed the hierarchical data model was quite prevalent thirty years ago
. Due to the recently increased importance of X.500/LDAP directories, which
are hierarchical, and the prevalence of aggregation hierarchies in datacub
es, there is now renewed interest in the hierarchical organization of data.
In this paper, we develop a framework for a modern hierarchical data model
, substantially improved from the original version by taking advantage of t
he lessons learned in the relational database context. We argue that this n
ew hierarchical data model has many benefits with respect to the ubiquitous
flat relational data model.