Page 180 - DCAP402_DCAO204_DATABASE MANAGEMENT SYSTEM_MANAGING DATABASE

Unit 10: Datalog and Recursion

Notes
Figure: 10.5: An Administrative DB with Media Data

(a) ER model of multiple media DB

(b) Relational model of multiple media DB
The relational model was quickly criticized for being ‘flat’ in the sense that all information is
represented as a set of tables with atomic cell values. The definition of well-formed relational
models requires that complex attribute types (hierarchic, composite, multi-valued, and derived)
be converted to atomic attributes and that relations be normalized. Inter-entity (inter-relation)
relationships are difficult to visualize in the resulting set of relations, making control of the
completeness and correctness of the model difficult. The relational model maps easily to the
physical characteristics of electronic storage media, and as such, is a good tool for design of a
physical database.
The entity-relationship approach to modelling, had two primary objectives: first to visualize
inter-entity relationships and second to separate the DB design process into two phases:
1. Record, in an ER model, the entities and inter-entity relationships required “by the
enterprise”, i.e. by the owner/user of the information system or application. This phase
and its resulting model should be independent of the DBMS tool that is to be used for
realizing the DB.

2. Translate the ER model to the data model supported by the DBMS to be used for
implementation.
This two-phase design supports data independence, i.e. the possibility of DB structure modification
at the physical level without requiring changes to the enterprise or user view of the DB content.

10.5 Specialization

We define specialization as a restriction on the extension of a process: a process p is a
1
specialization of a process p if every instance of p is also an instance of p , but not necessarily
0 1 0
vice versa. This definition can be restated to take into account frame of reference; there are two
cases to consider:
1. Both processes are described using the same frame of reference. In this case the extensions
of the processes are described in the same terms and can be compared directly. Thus p is
1
a specialization of p if and only if the extension of p as described using the given frame
0 1
of reference is a subset of the extension of p as similarly described.
0

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