31-08-2012, 03:58 PM
Ceng 302 Introduction to DBMS Basic Concepts
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Categories of data models
● Conceptual (high-level, semantic) data models:
Provide concepts that are close to the way many
users perceive data. (Also called entity-based or
object-based data models.)
e.g. ER model
● Implementation (representational) data models:
Provide concepts that fall between the above two,
balancing user views with some computer storage
details.
e.g. Relational model, OO model, network model
● Physical (low-level, internal) data models:
Provide concepts that describe details of how data
is stored in the computer.
History of Data Models
● Network Model: the first one to be implemented by
Honeywell in 1964-65 (IDS System). Adopted heavily due
to the support by CODASYL (CODASYL - DBTG report of
1971). Later implemented in a large variety of systems -
IDMS (Cullinet - now CA), DMS 1100 (Unisys), IMAGE
(H.P.), VAX -DBMS (Digital Equipment Corp.).
● Hierarchical Data Model: implemented in a joint effort by
IBM and North American Rockwell around 1965. Resulted
in the IMS family of systems. The most popular model.
Other system based on this model: System 2k (SAS inc.)
● Relational Model: proposed in 1970 by E.F. Codd (IBM),
first commercial system in 1981-82. Now in several
commercial products (DB2, ORACLE, SQL Server,
SYBASE, INFORMIX).
Network Model
• ADVANTAGES:
• Network Model is able to model complex relationships and
represents semantics of add/delete on the relationships.
• Can handle most situations for modeling using record types and
relationship types.
• Language is navigational; uses constructs like FIND, FIND
member, FIND owner, FIND NEXT within set, GET etc.
Programmers can do optimal navigation through the database.
• DISADVANTAGES:
• Navigational and procedural nature of processing
• Database contains a complex array of pointers that thread
through a set of records.
Little scope for automated "query optimization”
Hierarchical Model
• ADVANTAGES:
• Hierarchical Model is simple to construct and operate on
• Corresponds to a number of natural hierarchically organized
domains - e.g., assemblies in manufacturing, personnel
organization in companies
• Language is simple; uses constructs like GET, GET UNIQUE,
GET NEXT, GET NEXT WITHIN PARENT etc.
• DISADVANTAGES:
• Navigational and procedural nature of processing
• Database is visualized as a linear arrangement of records
• Little scope for "query optimization"
Three-Schema Architecture
• Defines DBMS schemas at three levels:
• Internal (physical) schema at the internal level to
describe physical storage structures and access paths.
Typically uses a physical data model.
• Conceptual schema at the conceptual level to describe
the structure and constraints for the whole database for a
community of users. Uses a conceptual or an
implementation data model.
• External schemas at the external level to describe the
various user views. Usually uses the same data model as
the conceptual level.