The Relational Model CS 186, Spring 2007, Lecture 2 Cow book Section 1.5, Chapter 3 Mary Roth

Administrivia Homework 0 –Due next Tuesday, Jan 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 Optimization and Execution Relational Operators Files and Access Methods Buffer Management Disk Space Management DB

Review: DBMS components Query Optimization and 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 Optimization and 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 Database app

Review: Typical architecture for DB applications DBMS Command lineGUI JDBC/ODBC app App server JDBC/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 Optimization and 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 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 Jones Smith Smith 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 (, … ) INSERT INTO ( ) VALUES ( ) DELETE FROM WHERE UPDATE SET = WHERE SELECT FROM WHERE

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 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 sidnameloginagegpa sidcidgrade 53666Carnatic101C 53666Reggae203B 53650Topology112A 53666History105B Enrolled Students PRIMARY Key FOREIGN 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 CS186A IMS requires –A change to add a field for CS186 –A change to Smith’s record to have him point to the new field

Keys are the key to data independence! sidnameloginagegpa sidcidgrade 53666Carnatic101C 53666Reggae203B 53650Topology112A 53666History105B Enrolled Students 53688CS186A 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 > Write a SQL query (DML) that withdraws $300 from Frodo’s account.