Chapter 2 dbChapter 2 dbChapter 2 dbChapter 2 dbChapter 2 dbChapter 2 db

1. 1
Chapter 2
DATABASE SYSTEM CONCEPT AND
ARCHITECTURE
• DBMS Architecture
– Tightly integrated systems architecture: the user
interface + DBMS + data are located in one place.
– Client-server system architecture:
User Interface DBMS + Data
Client Module Server Module
Request
Reply
Ala Hamarsheh --
ahamarsheh@aauj.edu

2. 2
• Data Model: a collection of concepts that used to
describe the structure of database.
 Structure: data type, constraints, and relationships.
– We use the data model to achieve some levels of data
abstraction.
– The data model should include a set of basic operations for
retrievals and updates on the DB.
• CATEGORIES OF DATA MODEL:
– High level: provide concepts that are close to the way
users perceive data (SQL).
– Low Level: provide concepts that describe the details
of how data is stored in the computer.
– Representational (implementation) data model:
provide concepts that may be understood by end
users but that are not too far from the way data is
organized within the computer (Entities)
Ala Hamarsheh --
ahamarsheh@aauj.edu

3. 3
• Schemas:
– Any data model should distinguish between the
description of the database and the database itself.
 The description of DB is called DB schema.
– Schema diagram: displays the structure of each
record type, BUT not the actual instances of
records.
StID SNAME CLASS MAJOR
STUDENT
Schema
Construct
This Schema will be
changed Infrequently
Ala Hamarsheh --
ahamarsheh@aauj.edu

4. 4
– Database State: the current set of instances or
occurrences.
 DB State will be changed frequently.
Notes:
 When we define new database we specify DB schema.
 Every time an update operation is applied to DB, we get
another DB state.
 DBMS responsible for ensuring that every state of the
database is a valid state  satisfies the structure and
constrains specified in the database schema.
 DBMS stores the description of the schema constructs
and constrains in the meta-data (catalog files).
 Schema is called intension whereas state is called
extension.
Ala Hamarsheh --
ahamarsheh@aauj.edu

5. 5
THREE SCHEMA ARCHITECTURE:
– Internal Level: to describe the data storage structure
and access paths-typically uses physical data model.
– Conceptual Schema: to describe the structure and
constraints for the whole database.-uses
implementational data model.
– External Level: to describe the various user views.
 We need mapping among these levels.
Ala Hamarsheh --
ahamarsheh@aauj.edu

6. 6
Ala Hamarsheh --
ahamarsheh@aauj.edu

7. 7
DATA INDEPENDENCE:
Changing the schema of one level of the DB system
doesn't require to change the schema of the next higher
levels.
Two types of data independence:
– Logical Data-Independence: the capacity to change
the conceptual schema without having to change the
external schema and their application programs.
– Physical Data-Independence: the capacity to
change the internal schema without having to change
the conceptual schema.
Ala Hamarsheh --
ahamarsheh@aauj.edu

8. 8
DBMS LANGUAGES:
– Data Definition language (DDL): used by DBA or
DB designers to specify the conceptual schema of a
DB.
• In many DBMSs the DDL is also used to define
internal and external schemas.
• In some DBMSs separate storage definition
language (SDL) and view definition language are
used to define internal and external schema.
– Data Manipulation Language (DML): used to
specify database retrievals and updates.
Ala Hamarsheh --
ahamarsheh@aauj.edu

9. 9
DML Commands can be:
1. Embedded in a general purpose programming
language (PL) like Java, Cobol, C++, .. Etc
2. Stand-alone, that applied directly (Query Language).
TWO TYPES OF DML:
– High-Level or Nonprocedural DML: DML
statements must be identified within the program so
that they can be extracted by the pre-compiler and
accessed by DMBS.
– Low-Level or Procedural DML: retrieves individual
records from DB and process each one separately.
 Use Looping
Ala Hamarsheh --
ahamarsheh@aauj.edu

10. 10
Ala Hamarsheh --
ahamarsheh@aauj.edu