Information Technology Reference
In-Depth Information
The CODASYL (Network) model is composed of two basic data constructs: the
record and the set, respectively. These two data constructs are built up from simpler
data elements which are discussed as follows:
• DataItem—Anoccurrenceofthesmallestunitofnameddata.Itisrepresentedin
the database by a value. A data item may be used to build other more complicated
data constructs. This corresponds to an attribute in the ER data model.
• DataAggregation—Anoccurrenceofanamedcollectionofdataitemswithina
record.
• Record—Anoccurrenceofanamedcollectionofdataitems or data aggregates.
This collection is in conformity with the record type definition specified in the
database schema.
• Set—Anoccurrenceofanamedcollectionofrecords.Asetoccurrenceisin
direct correspondence with the set type definition specified in the database sche-
ma. Each set type consists of one owner record type and at least one member
record type.
• Area—Thenotionofanareausedtoidentifythepartitionofrecordoccurrences.
An area is a named collection of records that need not preserve owner-member
relationships. An area may contain occurrences of one or more record types and
a record type may have occurrences in more than one area.
While designing a network database, the following rules must be followed to ensure
the integrity of the definitions:
• Anareaisanamedsubdivisionofthedatabase.
• Anarbitrarynumberofareasmaybedefinedinasystem.
• Recordsmaybeindependentlyassignedtoareasoftheirsetassociations.
• Arecordoccurrenceisstoredwithinoneareaonly.
• Asingleoccurrenceofasettypemayspanseveralareas.
• Eachsettypemustbeuniquelynamedandmusthaveanownerrecordtype. A
special type of set which has exactly one occurrence and for which the system is
the owner may be declared as a singular set.
• Anyrecordtypemaybedefinedastheownerofoneormoresettypes.
• Ifasethasanownerrecordwhichhasnomemberrecord,thesetisknownas
empty or null.
• Arecordcannotbeusedasanownerrecordinmorethanoneoccurrenceofthe
same set type.
• Arecordcannotbeparticipatedasamemberrecordinmorethanoneoccurrence
of the same type.
• Asetcontainsexactlyoneoccurrenceofitsowner.
• Asetmayhaveanynumberofmemberoccurrences.
The following shows some record entries and set entries of the university enroll-
ment system.
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