How to Retrieve Data from a Multifile Data Structure in a DBMS Model
A Multifile structure
describes multiple types of entities in separate logical files, that
is, an entry type corresponds to each entity class. All meaningful relationships
are formally defined to the system.
A Multifile data structure schema diagram for our sample database with all relationships represented explicitly in stored data item values within the entity record types. With the addition of a fifth record to store all valid employee-secondary skill associations, this becomes a multiple flat file data structure, commonly called a relational data model.
Approaches to querying a Multifile data structure
Just because a system
offers the ability to define and create Multifile data structure (often
called network or relational structures) is no guarantee that it also
provides a high-level language capability for querying such structures.
In fact, to do so is the exception in todays marketplace for database
A useful extension of this approach is to identify or deliver multiple record instances (of the same record type) in response to a single user request containing a Boolean selection expression.
Some systems have even made this record-level level language facility available to other online, interactive user-thus providing for online navigation. Some vendors have misleadingly called this a natural, user-oriented retrieval facility. Whatever the description, it is still a low-level approach to retrieving from a Multifile structure.
Another approach requires the user to view only a hierarchical data structure within the Multifile structure. This is possible by suppressing some relationships and assuming the rest to be hierarchical relationships. the perceived hierarchical structure may be multipath, but is more often only a single-path hierarchy. The user schema is an appropriate vehicle for defining a users hierarchical view within a Multifile data structure.
A variant of this approach allows the user to declare a different hierarchical view immediately before issuing the retrieval request in term of that view. Thus, the hierarchical view can change from one query to the next. Against such a restricted view of the database, the system can now provide a high-level retrieval language as described earlier. This is a common approach to providing some high-level language capability against a stored database which is actually a Multifile data structure.
Multifile retrieval language
A desirable goal
is to provide a high-level language which can reference two or more
files using a single language statement. Since there may be more than
one relationship between any two entity classes, the language must make
explicit provision for naming the relationship to be used as part of
the language statement. When only one relationship exists between two
entity classes referenced in a query, the system can unambiguously determine
what entity instances of one class are related to instances of the other
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