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

2. Today’s Lecture Overview on Database Management Systems
Course Logistics

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)
What’s inside a DBMS
Collection of interrelated data (E.g., for a given application)
Set of programs to secure and 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
Data stored on disk
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
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
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 platform that allows us to create, stores, use, and maintain a database
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 (ER) data model
Object-based data models (Object-oriented)
Semi-structured data model (XML)
Other older models:
Network model
Hierarchical model
We will learn these two models

11. Example: ER Model
Graphical model for describing entities, attributes, and relationships

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

13. Query Language (SQL)
Language for accessing and manipulating the data organized by the appropriate data model
SQL: Structured Query Language
SELECT ID, Name
FROM Student
WHERE address=“320FL”;

14. Query Language Two classes of languages
Procedural – user specifies what data is required and how to get those data
Declarative (non-procedural) – user specifies what data is required without specifying how to get those data
DBMSs use declarative language (SQL)
SELECT ID, Name
FROM Student
WHERE address=“320FL”;

15. A Big Picture of What You will Learn

16. You will Learn Data Model Data Schema Database
Relational Model
Entity-Relationship (ER) Model
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
Query Language
SQL Language

17. 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

18. Example Database : Relational
Tabular View of Data in Airline System
flightNo
start
destination
miles
101
BOS
LAX
3000
102
PVD
2900
Flight
pName
freqFlyerID
DoB
milesEarned
Mike
3433
1980
12000
Mary
5872
1981
11000
Passenger
flightNo
freqFlyerID
date
101
3433
Jan 4
102
5872
Jan 5
Travel
Tabular view of data is called “Relational Model”

19. Entity-Relationship Model: Overview
Models the application as a collection of entities and relationships
Represented using Entity-Relationship Diagram (ERD)

20. SQL: Overview
SQL: 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
ODBC: Open Database Connectivity
JDBC: Java Database Connectivity

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

22. 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
Usually hidden from users

23. Levels of Abstraction: Airline Application Example
Logical (Conceptual) Level
Flight, Passenger, Travel tables
Physical Level
Flight table stored as a sorted file on the flight number
Index on flightNo attribute for Flight relation
View Level (External Schema)
NoOfPassengers (flightNo, date, numPassengers)
Hide employees salary
These levels of abstraction lead to
“Data Independence”

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

25. Other Advanced 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

26. 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

27. 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
SELECT ID, Name
FROM Student
WHERE address=“320FL”;
Optimizer = “The Bread and Butter of a DBMS !”

28. 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)

29. 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

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

31. 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
DB research is one of the most exciting areas in Computer Science !!