In this chapter we address the question: What options are available within the relational model for implementing hierarchical data types? The special characteristics of these types are examined and the prospects for implementing them are assessed separately for both the current (SQL2) and proposed (SQL3) versions of the model. While the proposed version's user-defined types seem to provide a suitable mechanism, the current version leaves no option but to implement hierarchies as features of application programs rather than databases. The findings of an experimental implementation of a hierarchy are described and the requirements of a generalized solution for such implementations are specified. Strategies for accommodating a number of specialized tree structures within the generalized solution are suggested.
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