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Loan
Contracts
Customers
(actual record)
Lpointers
Copointers
(virtual record)
Fig. 2.2 m:n relationship of a hierarchical model in a loan system
Due to the limitation of an inverted tree structure, the hierarchical model cannot
be used to implement the following:
• m:nrelationship between two record types
• Achildrecordtypewithmorethanoneparentrecordtype
• n-aryrelationships with more than two participating record types
With virtual pointers architecture, each record type can be in an m:n relationship
with another record type through the pointers. The record type with the source point-
ers is called the logical child. Its target record type is called the logical parent. For
example, Fig. 2.2 shows that the record types of Cpointers and Lpointers are logical
child record types. Their corresponding logical parent record types are Customers
and Loan Contracts. As a result of these pointers, Record type Customers and Loan
Contracts are in an m:n relationship such that each customer can sign many loan
contracts, and each loan contract can be signed by many customers.
2.1.2
Hierarchical Data Manipulation Language (HDML)
HDML is a record-at-a-time language for manipulating hierarchical databases. The
commands of a HDML must be embedded in a general-purpose programming lan-
guage, called a host language. Following each HDML command, the last record ac-
cessed by the command is called the current database record. The system maintains
a pointer to the current record. Subsequent database commands proceed from the
current record, and move to a new current record depending on the operation of the
command. The traversal of the database access follows the inverted tree structure,
i.e., each database navigation path according to the hierarchical sequence. For ex-
ample, Fig. 2.1 has five access paths as follows:
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