Ch1- Introduction to dbms

CChhaapptteerr 11:: IInnttrroodduuccttiioonn

Database Management System
(DBMS)
Definition: DBMS contains information about a particular
enterprise, like-
Collection of interrelated data
Set of programs to access the data
An environment that is both convenient and efficient to use
Databases can be very large.
Database Applications:
Banking: transactions
Airlines: reservations, schedules
Universities: registration, grades
Sales: customers, products, purchases
Online retailers: order tracking, customized recommendations
Manufacturing: production, inventory, orders, supply chain
Human resources: employee records, salaries, tax
deductions
Example, University database.

Drawbacks of using file systems to
store data
Data redundancy and inconsistency
 Multiple file formats, duplication of information in
different files
Difficulty in accessing data
 Need to write a new program to carry out each new
task
Data isolation — multiple files and formats
Integrity problems
 Hard to add Integrity constraints (e.g., account
balance 0) or change existing ones
Atomicity of updates
 Failures may leave database in an inconsistent state with
partial updates carried out
 Example: Transfer of funds from one account to another
should either complete or not happen at all

Drawbacks of using file systems to store data
(Cont.)
Concurrent access by multiple users
 Concurrent access needed for performance
 Uncontrolled concurrent accesses can lead to inconsistencies
– Example: Two people reading a balance (say 100) and
updating it by withdrawing money (say 50 each) at the same
time
Security problems
 Hard to provide user access to some, but not all, data
Database systems offer solutions to all the above problems

Levels of Abstraction
Physical level: lowest level of abstraction. Describes how a
record (e.g., customer) is stored. Very much complex.
Logical level: next-higher level. Describes data stored in
database, and the relationships among the data.
type instructor = record
ID : string;
name : string;
dept_name : string;
salary : integer;
end;
View level: application programs hide details of data types.
Views can also hide information (such as an employee’s salary)
for security purposes.

View of Data
An architecture for a database system

Instances and Schemas
Similar to types and variables in programming languages
SScchheemmaa – the logical structure of the database
Example: The database consists of information about a set of
customers and accounts and the relationship between them
Analogous to type information of a variable in a program
Physical schema: database design at the physical level
Logical schema: database design at the logical level
Instance – the actual content of the database at a particular point in
time
Analogous to the value of a variable
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 Models
Definition: A collection of tools for describing
Data
Data relationships
Data semantics
Data constraints
Types of data models:
Relational model
Entity-Relationship data model (mainly for database
design)
Object-based data models (Object-oriented and Object-relational)
Semistructured data model (XML)
Other older models:
Network model
Hierarchical model

Relational Model
Example of tabular data in the relational model
Columns
Rows

Data Manipulation Language
(DML)
Language for accessing and manipulating the data organized
by the appropriate data model
DML also known as query language
Two classes of languages
Procedural – user specifies what data is required and
how to get those data
Declarative (nonprocedural) – user specifies what
data is required without specifying how to get those data
SQL is the most widely used query language

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) of a
database. It has-
Database schema
Integrity constraints
 Primary key (ID uniquely identifies instructors)
 Referential integrity (references constraint in SQL)
Authorization

The Entity-Relationship Model
Models an enterprise as a collection of entities and
relationships
Entity: a “thing” or “object” in the enterprise that is
distinguishable from other objects
 Described by a set of attributes
Relationship: an association among several entities
Represented diagrammatically by an entity-relationship
diagram:
What happened to dept_name of instructor and student?

Storage Management
Storage manager is a program module that provides the
interface between the low-level data stored in the database
and the application programs and queries submitted to the
system.
The storage manager is responsible to the following tasks:
Interaction with the file manager
Efficient storing, retrieving and updating of data
Provides:
Storage access
File organization
Indexing and hashing

Query Processing
1. Parsing and translation
2. Optimization
3. Evaluation

Query Processing (Cont.)
Alternative ways of evaluating a given query
Equivalent expressions
Different algorithms for each operation
Cost difference between a good and a bad way of evaluating a
query can be enormous
Need to estimate the cost of operations
Depends critically on statistical information about relations
which the database must maintain
Need to estimate statistics for intermediate results to
compute cost of complex expressions

Transaction Management
A transaction is a collection of operations that performs a
single logical function in a database application
Transaction-management component ensures that the
database remains in a consistent (correct) state despite
system failures (e.g., power failures and operating system
crashes) and transaction failures.
Concurrency-control manager controls the interaction
among the concurrent transactions, to ensure the
consistency of the database.

Database Architecture
The architecture of a database systems is greatly influenced by
the underlying computer system on which the database is
running:
Centralized
Client-server
Parallel (multi-processor)
Distributed

Ch1- Introduction to dbms

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Ch1- Introduction to dbms