The Relational ModelCS 186, Spring 2007, Lecture

2Cow book Section 1.5,

Chapter 3Mary Roth

Administrivia

• Homework 0 – Due next Tuesday, Jan 23 10 p.m.– Submission instructions added to

homework description– Class account forms here if you need

them

• Discussion sections will meet today

• Questions?

Outline

• What we learned last time– What is and what good is a DBMS

anyway?– Components of a DBMS

• New stuff– A brief history of databases– The relational data model

Review: What is a database?

• A collection of data organized for rapid search and retrieval– Data collection has some logical

meaning, and some reason for it to be organized in a particular way.

Review: What is a DBMS?

• A software system designed to manage a database.– Think big and lots of data

• 300,000,000 bank accounts

– Think mission critical• 1,000,000 transactions a day

• You’d need a DBMS to:– Help you find things fast– Help you keep track of what’s going on

Review: ACID properties

• A DBMS ensures a database has ACID properties:

• Atomicity – nothing is ever half baked; database changes either happen or they don’t.

• Consistency – you can’t peek at the data til it is baked; database changes aren’t visible til they are commited

• Isolation – concurrent operations have an explainable outcome; multiple users can operate on a database without conflicting

• Durability – what’s done is done; once a database operation completes, it remains even if the database crashes

Review: DBMS components• A DBMS is like an ogre; it has layers

– We’re going to learn about these layers all semester– We’re going to build several layers in our homework projects

Query Optimizationand Execution

Relational Operators

Files and Access Methods

Buffer Management

Disk Space Management

DB

Review: DBMS components

Query Optimizationand Execution

Relational Operators

Access Methods

Buffer Management

Disk Space Management

DB

•Makes efficient use of disk space

-> Think 300,000,000 accounts!

•Makes efficient use of RAM

-> Think 1,000,000 simultaneous requests!

•Provides generic ways to combine data

-> Do you want a list of customers and accounts or the total account balance of all customers?

•Figures out the best way to answer a question

-> There is always nore than 1 way to skin a cat…!

•Provides efficient ways to extract data

-> Do you need 1 record or a bunch?

Database application

•Talks to DBMS to manage data for a specific task

-> e.g. app to withdraw/deposit money or provide a history of the account

Review: How does a DBMS work?

Query Optimizationand Execution

Relational Operators

Access Methods

Buffer Management

Disk Space Management

Customer accounts stored on disk

Query in:e.g. “Select min(account balance)”

Data out:e.g. 2000

Database app

Review: Typical architecture for DB applications

DBMS

Command line GUI JDBC/ODBC app

App serverJDBC/ODBC

Web browser

1. Enter queries, etc.By typing text

2. Graphically compose queries, look at data

3. Embed database access in a program

4. Embed database access in a web application

Summary: Benefits of a DBMS

1. Data independence– applications worry about what data they want, not

how it is stored

2. Efficient data access– DBMS is smart about how to retrieve data

3. Data integrity and security– DBMS won’t let you corrupt data

4. Centralized administration– stored data on single server and let people

specialize in managing it

5. Concurrent access– Handles multiple users efficiently and recoverably

6. Reduced application development time – Derived from 1-5

Minibase is a Java-based DBMS

Query Optimizationand Execution

Relational Operators

Access Methods

Buffer Management

Disk Space Management

DB

•Provided for you

•Homework 1

•Homework 4

•Homework 3 (Pencil-work)

•Homework 2

Database application •Homework 5

Intermission

• Get up and stretch

• Ask a quick question

• Get a drink of water

A brief history of databases

• Birth of the DBMS parallels adoption of computer over 1960s and 1970s

• 1960s: IBM introduced IMS– 36 years old!– ‘Legacy’ technology, but still important!

• 100,000,000 bank transactions a day move money through IMS system

• A bank manages over 300,000,000 online bank accounts on IMS

• One production IMS system has been running for over 8 years without down time or a crash

A brief history of databases

• 1970: Ted Codd introduced the relational data model – Revolutionary idea that spurred a

flurry of DBMS activity– …at IBM (System R, DB2)– …at Universities like Berkeley (Ingres)– …at Oracle (it was born!!)

• Ted Codd won the Turing award in 1981

• Larry Ellison became a gillionaire

So what’s the big deal about the relational data model?

• What is the first benefit of a DBMS?

– Data independence

• A Data Model is key to data independence – It’s the link that provides

an abstraction between user’s view of the world and bits stored in computer 1010111

101

Student (sid: string, name: string, login: string, age: integer, gpa:real)

So what’s the big deal about the relational data model?

• It is now the most widely used data model.

• Before 1970, there were other data models…– Network– Hierarchical (IMS)

• But they didn’t really provide data independence– If the data layout changed, the application had to change– If you wanted to change the layout, you often had to bring

the whole system down– Changes had to occur over scheduled system down time.

• Slow! Annoying! Expensive!

• The relational model changed all that.

Relational Database: Definitions

• Relational database: a set of relations. • Relation: made up of 2 parts:

– Schema : specifies name of relation, plus name and type of each column.

• e.g. Students(sid: string, name: string, login: string, age:

integer, gpa: real)

– Instance : a table, with rows and columns. • #rows = cardinality• #fields = degree / arity

• You can think of a relation as a set of rows or tuples. (It’s basically a spread sheet!)– i.e., all rows are distinct

Ex: Instance of Students Relation

sid name login age gpa

53666 Jones jones@cs 18 3.4 53688 Smith smith@eecs 18 3.2 53650 Smith smith@math 19 3.8

• Cardinality = 3, arity = 5 , all rows distinct

• Do all values in each column of a relation instance have to be distinct?

SQL - A language for Relational DBs

• SQL (a.k.a. “Sequel”), standard language

• Data Definition Language (DDL)– create, modify, delete relations– specify constraints– administer users, security, etc.

• Data Manipulation Language (DML)– Specify queries to find tuples that

satisfy criteria– add, modify, remove tuples

SQL Overview

• CREATE TABLE <name> ( <field> <domain>, … )

• INSERT INTO <name> (<field names>) VALUES (<field values>)

• DELETE FROM <name> WHERE <condition>

• UPDATE <name> SET <field name> = <value> WHERE <condition>

• SELECT <fields> FROM <name> WHERE <condition>

Creating Relations in SQL

• Creates the Students relation.– Note: the type of each field is specified,

and enforced by the DBMS whenever tuples are added or modified.

CREATE TABLE Students(sid CHAR(20), name CHAR(20), login CHAR(10), age INTEGER, gpa FLOAT)

Table Creation (continued)

• Another example: the Enrolled table holds information about courses students take.

CREATE TABLE Enrolled(sid CHAR(20), cid CHAR(20), grade CHAR(2))

Adding and Deleting Tuples

• Can insert a single tuple using:

INSERT INTO Students (sid,name,login,age,gpa) VALUES (‘53688’,‘Smith’,‘smith@ee’,18,3.2)

• Can delete all tuples satisfying some condition (e.g., name = Smith):

DELETE FROM Students S WHERE S.name = ‘Smith’

Powerful variants of these commands are available; more later!

Keys

• Keys are a way to associate tuples in different relations

sid name login age gpa53666 Jones jones@cs 18 3.453688 Smithsmith@eecs 18 3.253650 Smithsmith@math 19 3.8

sid cid grade53666 Carnatic101 C53666 Reggae203 B53650 Topology112 A53666 History105 B

Enrolled Students

PRIMARY KeyFOREIGN Key

Keys are the key to data independence!

• Big improvement over the hierarchical model– Relationships are determined by field

value, not physical pointers!

Keys are the key to data independence!

• Let’s enroll Smith in EECS in CS186– With hierarchical model

CS186 A

• IMS requires – A change to add a field for CS186

Smith smith@eecs18 3.253688

– A change to Smith’s record to have him point to the new field

Keys are the key to data independence!

sid name login age gpa53666 Jones jones@cs 18 3.453688 Smithsmith@eecs 18 3.253650 Smithsmith@math 19 3.8

sid cid grade53666 Carnatic101 C53666 Reggae203 B53650 Topology112 A53666 History105 B

Enrolled Students

53688 CS186 A

• Let’s enroll Smith in EECS in CS186– With relational model

• Relation model only requires – A data change to add a new row to Enrolled table

Let’s return to our bank…

• Can we apply a relational model to our bank spreadsheet?

Exercises to test your understanding…

• Write the DDL for our bank tables.– Include primary and foreign key

definitions

• Write a SQL query (DML) that returns the names and account balances for all customers that have an account balance > 2500.

• Write a SQL query (DML) that withdraws $300 from Frodo’s account.