Mapping Of Networks To File : DBMS

Mapping  Of Networks To File


We implement links by adding pointer fields to records that are
associated via a link

-> Each record must have one pointer field for each link with which
it is associated.
-> Example data-structure diagram and corresponding database.




* Since the depositor link is many to many, each record can be
associated with an arbitrary number of records (e.g., the account
record would have a pointer to the customer record for each
customer who has that account).

-> Direct implementation of many-to-many relationships requires
the use of variable length records.

-> The DBTG model restricts links to be either one to one or one to
many; the number of pointers needed is reduced, and it is
possible to retain fixed-length records.




* Assume that the depositor link is one to many and is represented
by the DBTG set depositor and this corresponding sample
database.

                   set name is depositor
                   owner is customer
                   member is account




* Because an account record can be associated with only one
customer record, we need only one pointer in the account record
to represent the depositor relationship.

-> A customer record can be associated with many account
records.

-> Rather ant using multiple pointers in the customer record, we can use a ring structure to represent the entire occurrence of the
DBTG set depositor.

-> In a ring structure, the records of both the owner an member
types for a set occurrence are organized into a circular list.

-> There is one circular list for each set occurrence (that is, for each
record of the owner type)