Hierarchical Databases

The traditional answer to some of these issues was the hierarchical database model. A hierarchical database is a series of flat files, each one similar to a spreadsheet, that are linked in structured treelike relationships (see Figure 11.2). Data are represented as a series of parent/child relationships. A patient's record (the parent) might link to "follow-up exam" children, and each of these children might link to the records of specialized procedures (grandchildren).

Each child segment can be linked to only one parent and a child can only be reached through its parent. This could create a problem. The radiology department might want to have a patient's X-ray results as its "children." While we would want to keep them with the appropriate set of follow-ups. Or perhaps store each exam as part of a master patient record. Will we need to make two or even three copies of the exam results?

To avoid data redundancy, all information in a hierarchical database is stored in a single location and referenced by links or physical pointers in other locations. For example, the "patients" record might contain actual data in the "specialized exam" segment while the "radiology" record held only a pointer to the "specialized exam" data in "patients."

On the down side, the link established by the pointers is permanent and cannot be modified. A design originally optimized to work

28The lack of an audit trail would meet with fatal objections from most regulatory agencies.

FIGURE 11.2 Hierarchical Database.

with the data in one way may prove totally inefficient in working with the data in other ways. The physical links make it very difficult to expand or modify the database; changes typically require substantial rewriting efforts and risk introducing errors and destroying irreplaceable data.

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