1. Introduction Instructor: Mohamed Eltabakh meltabakh@cs.wpi.edu
CS3431 – Database Systems I
Introduction
Instructor: Mohamed Eltabakh
CS3431-B11

2. Today’s Lecture Overview on Database Management Systems
Course Logistics
Entity-Relationship Data Model

3. What is a Database System?
Software platform for managing large amounts of data
Managing means:
Storing, querying, indexing, and structuring the data
Different names refer to the same thing:
Database systems
Database management systems
DBMS

4. What is a Database System? (Cont’d)
DBMS contains information about a particular enterprise (application)
Collection of interrelated data
Set of programs to access the data
An environment that is both convenient and efficient to use
Usually data is too large to fit in computer memory at once
Usually many users want to access this data and do so fast
Databases touch all aspects of our lives. We use it without knowing !!!

5. Database Applications
Have you ever used a database application?
E-commerce: books, equipment etc. at Amazon
Banks -- your valuable $$ and ATM transactions
Airlines – manage flights to get you places
Universities – manage student enrollment
GIS (Maps) – find restaurants closest to WPI
WWW (World Wide Web) – blobs, wikis, etc.
Bio-informatics (genome data) ?
Data is everywhere. To efficiently manage it, we need DBMS

6. Why use DBMS, and not files?
Several drawbacks of using file systems to store data
Data redundancy and inconsistency
Multiple file formats, duplication of information in different files
Multiple records formats within the same file
No order enforced between fields
Difficulty in accessing data
Need to write a new program to carry out each new task
No indexes, always scan the entire file
Integrity problems
Modify one file (or field in a file), and not changing the dependent fields or files
Integrity constraints (e.g., account balance > 0) become “buried” in program code rather than being stated explicitly

7. Why use DBMS, and not files? (Cont’d)
Concurrent access by multiple users
Many users need to access/update the data at the same time (concurrent access)
Uncontrolled concurrent access can lead to inconsistencies
Example: Two people are updating the same bank account at the same time
Security problems
Hard to provide user access to some, but not all, data
Recovery from crashes
While updating the data the system crashes
Maintenance problems
Hard to search for or update a field
Hard to add new fields

8. DBMS Provides Solutions
Data consistency even with multiple users
Efficient access to the data
Data integrity embedded in the DBMS
Recovery from crashes, security

9. Basic Terminology Data Model Data Schema Database
Tools used for describing the data
Data Schema
Describes structures for a particular application, using the given model
Database
Collection of actual data that conforms to given schema
Database Management System (DBMS)
Software that allows us to create, stores, use, and maintain a database (conforming to given model)
Data Manipulation Language (DML)
Language to manipulate, e.g., update or query, the data

10. Data Model A collection of tools for describing Several data models:
Data relationships
Data semantics
Data constraints
Several data models:
Relational model
Entity-Relationship (ERD) 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
We will learn these two models

11. Data Schema
Captures the relationships between objects (“entities”) in the application
Schemas can be represented graphically or textual

12. 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
DBMSs use declarative language
SQL (Structured Query Language ) is the most widely used declarative query language in DBMSs
We will learn SQL

13. A Big Picture of What You will Learn
Data Model
Relational Model
Entity-Relationship (ERD)
Data Schema
How to put pieces together to build a schema describing the application
Database
Build an actual database and manipulate data
Database Management System (DBMS)
We will use Oracle
Data Manipulation Language (DML)
SQL Language

14. Relational Data Model: Overview
The most widely used model today
It is a tabular representation of the data
Main concepts:
Relations (Tables), basically a table with rows and columns.
Every relation has a schema, which describes the columns, or fields.
Field or attribute 5

15. Example Database : Relational
Tabular View of Data in Airline System
flightNo
start
destination
miles
101
BOS
LAX
3000
102
PVD
2900
Flight
pName
ffNumber
DoB
milesEarned
Joe
1001
1980
12000
Mary
1002
1981
11000
Passenger
flightNo
ffNumber
date
101
1001
Jan 4
102
1002
Jan 5
Travel
Tabular view of data is called Relational Model

16. Entity-Relationship Model: Overview
Models the application as a collection of entities and relationships
Represented using Entity-Relationship Diagram (ERD)
Main Concepts:
Entities: each entity describe an “object” or “thing” that has some characteristics
Each entity has attributes associated with it
There are relationships between the entities
Attributes
Relationship
Entities

17. SQL: Overview
SQL: Widely used non-procedural language to access the data inside a database
External programs, e.g., in C or Java, typically access the database using:
Language extensions to allow embedded SQL
Interfaces, e.g., ODBC or JDBC, that allow connection to a database and issuing SQL commands

18. Logical vs. Physical
How this information is stored???

19. Levels of Abstraction

20. Levels of Abstraction View Level --describes how users see the data
Logical Level – describes the logical structures used
Relational Model
ERD model
Physical Level -- describes files and indexes

21. Levels of Abstraction: Example
Logical (Conceptual) Level
Flight, Passenger, Travel tables
Physical Level
Flight table stored as a sorted file
Index on flightNo attribute for Flight relation
View Level (External Schema)
NoOfPassengers (flightNo, date, numPassengers)
Hide employees salary

22. Physical Data Independence
DBMS has the three levels of abstractions
Ability to modify the physical schema without modifying the logical or view schema
Logical data independence:
Logical schema can change, but views need not change
Protection from changes in logical structure of data
Physical data independence:
Physical schema such as indexes can change, but logical schema need not change
Protection from changes in physical structure of data

23. Other Aspects of DBMSs Efficient access Query optimization
Concurrency control
Recovery control
>> We will not have time to study these subjects during the course
>> It is important to know their existence and what is meant by each component

24. Efficient Access Indexing :
Indexes gives direct access to “necessary” portion of data, as opposed to sequential access in files
Directly find this customer without scanning all customers

25. Query Optimization Costing: Query optimization :
Estimate expected execution times
Query optimization :
Generates many alternatives to answer a query
Estimates the cost of each alternative
Automatically determine and prepare optimal (or near optimal) access plans for getting the data
Optimizer = “The Bread and Butter of a DBMS !”

26. Concurrency Control
DBMS ensures data is consistent under concurrent access
E.g.: multiple airline staff trying to reserve a seat for different customers
Concepts:
Transactions – grouping multiple instructions (reads/writes) into one atomic unit
Locks – locking of resources (tables)

27. Recovery Control
If system crashes in middle of transaction, recovery must be provided :
Cannot afford to loose data or leave it inconsistent
Concepts:
Logging of transactions’ actions
Ability to redo or undo transactions

28. Who uses databases? End users DB application programmers
Database Administrators
Database design
Security, Authorization
Data availability, crash recovery
Database tuning (for performance)

29. Summary : Why study DBMS?
Need to process large amounts of data efficiently
Video, WWW, computer games, geographic information systems (GIS), genome data, digital libraries, etc.
Make use of all functionalities provided by DBMSs
DB administrators and programmers hold rewarding jobs
DBMS research is one of the most exciting areas in Computer Science !!