Database Reference
In-Depth Information
The hierarchical data model, however, has been derived from IBM's Information
Management System (IMS) database that has been in wide use. In an IMS data-
base, the data structures are placed in a hierarchical top-down arrangement.
Summary of Basic Concepts
Data is organized in the form of data segments being arranged as an inverted
tree.
Two fundamental modeling concepts make up the hierarchical data model:
segment types and parent-child relational types.
Segments are linked in parent-child relationships, as a top-down structure.
A child segment is linked to just one parent segment whereas a parent segment
may be linked to many child segments.
A single segment, called the root segment, exists at the top of the inverted
tree. The root segment does not participate as a child in any parent-child
relationship.
All other segments participate in relationships as child segments.
A segment without any child segments is known as a leaf segment.
For any segment type in the hierarchical tree, a single path exists from the
root.
Each segment type generally represents an entity type of the organization.
Data fields in the record types denote the attributes of the entity type.
Logical links between related segments are implemented through physical
addresses (pointers) in the segment itself.
Semantic to Hierarchical Data Model
The following indicates the mapping of components of the semantic data model into
components of the hierarchical data model.
Object Sets
Object set is transformed into segment type.
The name of the object set becomes the name of the record type.
CUSTOMER object is transformed into CUSTOMER segment type.
The object instances of an object set become the occurrences of the trans-
formed segment type.
The complete set of object instances becomes the total set of segment
occurrences.
Attributes
Attributes of an object are transformed into the data fields of the record type.
The names of the attributes become the names of the data fields.
Domain of values of each attribute becomes the domain for the corresponding
data field.
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