Database Management System Lecture SlideCh-2.ppt

Chapter 2: Database System Models
and Architecture
Fundamentals of Database Systems
for 2nd year Computer Science students.
Prepared by: Gebriye Embafresu
- Huawei HCIA Security Certified
- Huawei HCIA Routing and Switching Certified
- Huawei HCIP Routing and Switching Certified
Computer Science Department
E-mail: gebriye14@gmail.com

(C) 2013, Gebriye E. Dept. Computer Science HCIA Security, HCIA R&S and HCIP R&S Certified
2.1 Data Models, Schemas, and
Instances
• Data Model: A set of concepts to describe the
structure of a database, and certain constraints
that the database should obey. They provide data
abstraction.
• Structure: data type, relationships,
constraints
• It is a collection of conceptual tools for describing
data, data relationships, data semantics, and
consistency constraints.
6-2

2.1 Data Models, Schemas, and
Instances
 Relational model
 Entity-Relationship data model (mainly for database design)
 Object-based data models (Object-oriented and Object-relational)
 Semi-structured data model (XML)
 Other older models:
• Network model
• Hierarchical model
6-3

Relational Model
• All the data is stored in various tables.
• Example of tabular data in the relational model
Columns
Rows
Ted Codd
Turing Award 1981

A Sample Relational Database

View of Data
An architecture for a database system

Instances and Schemas
• Similar to types and variables in programming languages
• Logical Schema – the overall logical structure of the database
– Example: The database consists of information about a set of
customers and accounts in a bank and the relationship between them
• Analogous to type information of a variable in a program
• Physical schema – the overall physical structure of the database
• Instance – the actual content of the database at a particular point in time
– Analogous to the value of a variable

Physical Data Independence
• Physical Data Independence – the ability to modify the physical schema
without changing the logical schema
– Applications depend on the logical schema
– In general, the interfaces between the various levels and components
should be well defined so that changes in some parts do not seriously
influence others.

Data Definition Language (DDL)
• Specification notation for defining the database schema
Example: create table instructor (
ID char(5),
name varchar(20),
dept_name varchar(20),
salary numeric(8,2))
• DDL compiler generates a set of table templates stored in a data
dictionary
• Data dictionary contains metadata (i.e., data about data)
– Database schema
– Integrity constraints
• Primary key (ID uniquely identifies instructors)
– Authorization
• Who can access what

Data Manipulation Language (DML)
• Language for accessing and updating the data organized by the appropriate data model
– DML also known as query language
• There are basically two types of data-manipulation
language
– Procedural DML -- require a user to specify what data are
needed and how to get those data.
– Declarative DML -- require a user to specify what data are
needed without specifying how to get those data.
• Declarative DMLs are usually easier to learn and use
than are procedural DMLs.
• Declarative DMLs are also referred to as non-procedural
DMLs
• The portion of a DML that involves information retrieval
is called a query language.

SQL Query Language
• SQL query language is nonprocedural. A query takes as input several
tables (possibly only one) and always returns a single table.
• Example to find all instructors in Comp. Sci. dept
select name
from instructor
where dept_name = 'Comp. Sci.'
• SQL is NOT a Turing machine equivalent language
• To be able to compute complex functions SQL is usually embedded in
some higher-level language
• Application programs generally access databases through one of
– Language extensions to allow embedded SQL
– Application program interface (e.g., ODBC/JDBC) which
allow SQL queries to be sent to a
database

Database Access from Application Program
• Non-procedural query languages such as SQL are not as powerful as a
universal Turing machine.
• SQL does not support actions such as input from users, output to displays,
or communication over the network.
• Such computations and actions must be written in a host language, such
as C/C++, Java or Python, with embedded SQL queries that access the
data in the database.
• Application programs -- are programs that are used to interact with the
database in this fashion.

Database Architecture
• Centralized databases
– One to a few cores, shared memory
• Client-server,
– One server machine executes work on behalf of multiple client
machines.
• Parallel databases
– Many core shared memory
– Shared disk
– Shared nothing
• Distributed databases
– Geographical distribution
– Schema/data heterogeneity

Database Architecture
(Centralized/Shared-Memory)

Database Applications
• Two-tier architecture -- the application resides at the client machine,
where it invokes database system functionality at the server machine
• Three-tier architecture -- the client machine acts as a front end and does
not contain any direct database calls.
– The client end communicates with an application server, usually
through a forms interface.
– The application server in turn communicates with a database
system to access data.
Database applications are usually partitioned into two or three parts

Two-tier and three-tier architectures

Database Users

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