Download Schema - Department of Computer Science

CS157A Lecture 2
DB Mangement Systems
Prof. Sin-Min Lee
Department of Computer
Science
San Jose State University
Objectives
In this Lecture you will learn:
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Purpose of Database Systems
View of Data
Data Models
Data Definition Language
Data Manipulation Language
Transaction Management
Storage Management
Database Administrator
Database Users
Overall System Structure
1. Database
Theory
Why use database?
Data is a valuable corporate resource which needs
adequate, accuracy, consistency and security
controls.
The centralised control of data means that for many
applications the data will already exist, and
facilitate quicker development.
Data will no longer be related by application
programs, but by the structure defined in the
database.
Easier to maintain systems
First generation
database systems
The network and hierarchical
databases of the 1970’s
 The first systems to offer DBMS
function in a unified system
 e.g. CODASYL, IMS
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Second generation
database systems
relational databases of the 1980’s
 data independence and non
procedural data manipulation language
 e.g. DB2, INGRES, NON-STOP SQL,
ORACLE, Rdb/VMS
 Focused on business data processing
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Third generation
database systems
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Problems with 2nd generation DBS
– inadequate for a broader class of applications (than
business data processing)
– e.g. CAD, CASE, Hypertext,
– storing text segments, graphics, etc is usually difficult in
2nd gen. systems
– Does not support complex data (folders)
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Most vendors are working on functional
enhancements on their 2nd gen. systems
Surprising degree of consensus on these features
3rd gen systems includes the desired capabilities of
next generation database systems
DATABASE AND DBMS
CONCEPTS
Database – is a collection of data, typically
describing the activities of one or more
related organizations.
A Database Management System DBMS) is
software designed to assist in maintaining
and utilization large collections of data.
The SQL (Structured Query Language),
developed by IBM is now the standard
query language.
ADVANTAGES OF A DBMS
• Data independence
Application programs should not, ideally, be exposed to details of
data representation and storage.
• Efficient Data access
A DBMS uses several powerful functions to store and retrieve
data efficiently.
Data Integrity and Security
The DBMS enforces integrity constraints to get a kind of
protection against prohibited access to data.
ADVANTAGES OF A DBMS
• Data Administration
When any users share the data, centralizing the administration
of data can offer significant improvements.
• Concurrent Access and Crash Recovery
A DBMS schedules concurrent access to the data in such
manner that users can think of the data as being accessed by
only one user at a time. DBMS also protects users from the
effects of system failures.
• Reduced Application Development Time
DBMS includes several important functions that are common to
many applications accessing data in the DBMS. In conjunction
with the high-level interface to the data, facilitates quick
application development.
DATA MODEL CONCEPT
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A data model is a collection of high-level data
description constructs that hide many low-level
storage details.
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Most database management systems today are
based on the Relational data model.
THE RELATIONAL MODEL
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The central data description construct is this model
is a Relation, which can be thought of a set of
records.
A Schema for a relation specifies:
– Name of relation (e.g. Students)
– Name of each field (or attribute or column)
– Type of each field.
Schema
Sudents (sid:string,age:integer)
Example of an Instance
of Students
Students
Sid
Age
53-666
28
53-688
35
Schema ------> Students (sid : String, Age : Integer)
Characteristics
Each row in the Students relation is a
record that describes any student.
 Integrity constraints are conditions that
the records must satisfy. E.g. sid is
unique.
 Oracle uses relational( and also
object) data model.
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Levels of Abstraction in a
DBMS
1.
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Conceptual Schema : or logical
schema describes all relations that
are stored in the database.
In the university example, these
relations contain information about
entities, such as students and
faculty, and about relationships,
such as students’ enrollment in
courses.
Conceptual Schema
For university, a conceptual schema
is:
– Students(sid:string,Age:integer)
– Faculty (fid: string, salary: real)
– Courses (cid: string, cname: string,
credits:integer)
Physical Schema
2.
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Physical Schema : specifies
additional storage details.
It summarizes how the relations
described in the conceptual schema
are stored on secondary storage
devices such as disks and tapes.
Creation of data structures called
indexes, to speed up data retrieval
operations
Physical Schema
A sample physical schema for the
university:
– Store all relations as unsorted files of
records
– Create indexes on the first column of
Students, Faculty, and Courses
relations.
External Schema
3.
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Each external schema consists of a
collection of one or more views and
relations from the conceptual
schema.
A view is conceptually a relation, but
the records in a view are not stored
in the DBMS.
A user creates any view from data
already stored.
External Schema
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For example: we might want to allow
students to find out the names of
faculty members teaching courses.
– This is the view associated:
Courseinfo (cid:string, fname:string)
– A user can treat a view just like a relation
and ask questions about the records in
the view, even though the records in the
view are not stored explicitly.
Database Management
System (DBMS)
•A software package such as Oracle or MS-Access.
•Manages data and relationships in the database.
•Creates a Data Dictionary to store Metadata – data about data.
•Manages all day-to-day transactions.
•Provides user with data independence at application level.
•Transforms logical data requests to match physical data structures.
•Secures access through passwords, restricted user access, and encryption.
•Provides backup and recovery mechanisms.
•Provides export and import utilities.
•Allows sharing of data with locking capabilities.
Traditional File Systems
In the the past as new applications were written they used existing
files, or created a new file for their use.
Sometimes several existing files need to be sorted and merged to
obtain the new file.Thus, it is probable that several files will contain the
same information stored in different ways. In other words, there will be
redundant and possibly inconsistent data.
Consider the files for an insurance company
POLICY#
PREMIUMS
AGENCY
POLICYHOLDER
data ADDRESS
PREMIUM-PA
PREMIUM-TOTAL
POLICY#
POLICYHOLDER
data ADDRESS
AGENT-CODE
RENEWAL-DATE
RENEWAL-AMT
Traditional File Systems
Applications were often considered in relative
isolation.
Data that should have been together was not.
The potential for flexible enquiry and reporting
was limited.
All validations were in the programs.
Procedures were required to for backup and
recovery.
All programmers had access to all records.
There was limited concurrent access.
Basic Definitions
Database: A collection of related data
Data: Known facts that can be recorded
Schema: Some part of the real world about which data
is stored in the database.
Database Management System(DBMS):
A software package to facilitate the creation and
maintenance of a computerised database.
Degrees of Data
Independence
·
Device Characteristics
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Blocking Factors
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Data Access Organisation
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Physical Record Location
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Logical Views (Local)
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Virtual Data Items
· Virtual Records
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Data Value Characteristics
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Data Element Name Only
Logical vs Physical Data Independence
Application
Program
Local Views
GLOBAL
LOGICAL
DATABASE
DESCRIPTION
Physical
Files
Logical Data
Physical Data
Independence
Independence
Three Schema
Architecture
ANSI & ISO suggest that DBMS should
have three schemas
·
CONCEPTUAL SCHEMA - the global logical
model of the data and processing of the
enterprise. i.e community user view.
·
EXTERNAL SCHEMA(S) - the logical
application views of the CS. i.e individual user
views.
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INTERNAL SCHEMA - internal level storage
view.
Three Schema
Architecture
External
Schema 1
External
Schema 2
Conceptual
Schema
Internal
Schema
External
Schema n
The External Level
· Each user has a language through which
they access or see the database.
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For the programmer - COBOL etc, for
the end-user a query language or special
purpose language.
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All languages will contain a data sublanguage which may be tightly or loosely
coupled to the host language.
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DSLs generally contain a data
definition language DDL and data
manipulation language DML.
The Conceptual Level
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A representation of the entire information
content of the database.
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Defined with a Conceptual Schema
Language which does not represent any
storage or access details.
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Should include all security and integrity
rules and some suggest the CS should
describe the total enterprise including all
allowable processing.
The Internal Level
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Low level representation of entire database.
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Deals with stored records rather than
conceptual or external records.
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Stored records may differ in structure from
conceptual records and external records.
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The Internal Schema is still one level away
from physical records which are often called
pages or blocks
Inter-Related Data
CLAIMS
RENEWALS
D
B
M
S
AGENCY
Data related by structure
Flexible enquiry easier
QUERY
RENEWALS
CLAIMS AGENCY
Multiple Applications
LOCAL
VIEWS
DATABASE
AGENCY
CLAIMS
RENEWALS
Database Management
System (DBMS)
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Collection of interrelated data
Set of programs to access the data
DBMS contains information about a particular enterprise
DBMS provides an environment that is both convenient and
efficient to use.
Database Applications:
– Universities: registration, grades
– Banking: all transactions
– Sales: Airlines: reservations, schedules
– customers, products, purchases
– Manufacturing: production, inventory, orders, supply chain
– Human resources: employee records, salaries, tax
deductions
Databases touch all aspects of our lives
Purpose of Database
System
 In the early
days, database applications
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were built on top of file systems
Drawbacks of using file systems to store
data:
– Data redundancy and inconsistency
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Multiple file formats, duplication of information in
different files
– Difficulty in accessing data
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Need to write a new program to carry out each new
task
– Data isolation — multiple files and formats
– Integrity problems
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Integrity constraints (e.g. account balance > 0) become
part of program code
Hard to add new constraints or change existing ones
Purpose of Database
Systems (Cont.)
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Drawbacks of using file systems (cont.)
– Atomicity of updates
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Failures may leave database in an inconsistent state with
partial updates carried out
E.g. transfer of funds from one account to another should
either complete or not happen at all
– Concurrent access by multiple users
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Concurrent accessed needed for performance
Uncontrolled concurrent accesses can lead to
inconsistencies
– E.g. two people reading a balance and updating it at the same
time
– Security problems
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Database systems offer solutions to all the
above problems
Levels of Abstraction
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Physical level describes how a record (e.g.,
customer) is stored.
Logical level: describes data stored in
database, and the relationships among the
data.
type customer = record
name : string;
street : string;
city : integer;
end;
View level: application programs hide details
of data types. Views can also hide
information (e.g., salary) for security
purposes.
View of Data
An architecture for a database system
Instances and
Schemas
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Similar to types and variables in programming languages
Schema – the logical structure of the database
– e.g., 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
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A collection of tools for describing
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data
data relationships
data semantics
data constraints
Entity-Relationship model
Relational model
Other models:
– object-oriented model
– semi-structured data models
– Older models: network model and
hierarchical model
Entity-Relationship
Model
Example of schema in the entityrelationship model
Entity Relationship
Model (Cont.)
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E-R model of real world
– Entities (objects)
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E.g. customers, accounts, bank branch
– Relationships between entities
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E.g. Account A-101 is held by customer Johnson
Relationship set depositor associates customers with
accounts
Widely used for database design
– Database design in E-R model usually converted
to design in the relational model (coming up next)
which is used for storage and processing
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Relational Model
Example of tabular data in the relational
Attributes
model
Customerid
192-83-7465
customername
Johnson
customerstreet
Alma
customercity
accountnumber
Palo Alto
A-101
019-28-3746
Smith
North
Rye
A-215
192-83-7465
Johnson
Alma
Palo Alto
A-201
321-12-3123
Jones
Main
Harrison
A-217
019-28-3746
Smith
North
Rye
A-201
A Sample Relational
Database
Data Definition Language (DDL)
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Specification notation for defining the
database schema
– E.g.
create table account (
account-number char(10),
balance
integer)
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DDL compiler generates a set of tables
stored in a data dictionary
Data dictionary contains metadata (i.e., data
about data)
– database schema
– Data storage and definition language
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language in which the storage structure and access
methods used by the database system are specified
Usually an extension of the data definition language
Data Manipulation
Language (DML)
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Language for accessing and manipulating
the data organized by the appropriate data
model
– DML also known as query language
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Two classes of languages
– Procedural – user specifies what data is required
and how to get those data
– Nonprocedural – user specifies what data is
required without specifying how to get those data
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SQL is the most widely used query
language
SQL
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SQL: widely used non-procedural language
– E.g. find the name of the customer with customer-id
192-83-7465
select customer.customer-name
from customer
where customer.customer-id = ‘192-83-7465’
– E.g. find the balances of all accounts held by the
customer with customer-id 192-83-7465
select account.balance
from depositor, account
where depositor.customer-id = ‘192-83-7465’
and
depositor.account-number =
account.account-number
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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 Users
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Users are differentiated by the way they
expect to interact with the system
Application programmers – interact with
system through DML calls
Sophisticated users – form requests in a
database query language
Specialized users – write specialized
database applications that do not fit into the
traditional data processing framework
Naïve users – invoke one of the permanent
application programs that have been written
previously
– E.g. people accessing database over the web,
bank tellers, clerical staff
Database Administrator
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Coordinates all the activities of the
database system; the database
administrator has a good
understanding of the enterprise’s
information resources and needs.
Database administrator's duties include:
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Schema definition
Storage structure and access method definition
Schema and physical organization modification
Granting user authority to access the database
Specifying integrity constraints
Acting as liaison with users
Monitoring performance and responding to changes
in requirements
Transaction
Management
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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.
Storage Management
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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
Overall System Structure
Application Architectures
Two-tier architecture: E.g. client programs using ODBC/JDBC to
communicate with a database
Three-tier architecture: E.g. web-based applications, and
applications built using “middleware”