1The Relational ModelCS 186, Fall 2006, Lecture 2R & G, Chap. 3Review• Why use a DBMS? OS provides RAMand diskReview• Why use a DBMS? OS provides RAMand disk– Concurrency– Recovery– Abstraction, Data Independence– Query Languages– Efficiency (for most tasks)– Security– Data IntegrityGlossary• Byte• Kilobyte: 2^10 B• Megabyte: 2^20 B• Gigabyte: 2^30 B• Terabyte: 2^40 B– Typical video store has about 8 TB– Library of Congress is about 20TB– Costs you about $600 at PCConnection, will hold your family videos• Petabyte: 2^50 B– Internet Archive WayBack Machine is now about 2 PetaByte• Exabyte: 2^60 B– Total amount of printed material in the world is 5 Exabytes• Zettabyte: 2^70 B• Yottabyte: 2^80 BData Models• DBMS models real world•Data Model is linkbetween user’s view ofthe world and bits storedin computer• Many models exist• We will concentrate onthe Relational Model1010111101Student (sid: string, name: string, login:string, age: integer, gpa:real)Why Study the Relational Model?• Most widely used model.• “Legacy systems” in older models– e.g., IBM’s IMS• Object-oriented concepts merged in– “Object-Relational” model• Early work done in POSTGRES research project atBerkeley• XML features in most relational systems– Can export XML interfaces– Can embed XML inside relational fields2Relational Database: Definitions• Relational database: a set of relations.• Relation: made up of 2 parts:– Schema : specifies name of relation, plus nameand 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• Can think of a relation as a set of rows ortuples.– i.e., all rows are distinctEx: Instance of Students Relationsid 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”), standardlanguage• Data Definition Language (DDL)– create, modify, delete relations– specify constraints– administer users, security, etc.• Data Manipulation Language (DML)– Specify queries to find tuples that satisfycriteria– add, modify, remove tuplesSQL 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 (domain) of each field isspecified, and enforced by the DBMSwhenever 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 tableholds information about coursesstudents take.CREATE TABLE Enrolled(sid CHAR(20), cid CHAR(20), grade CHAR(2))3Adding 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 indifferent relations• Keys are one form of integrity constraint(IC)sid name login age gpa53666 Jones jones@cs 18 3.453688 Smith smith@eecs 18 3.253650 Smith smith@math 19 3.8sid cid grade53666 Carnatic101 C53666 Reggae203 B53650 Topology112 A53666 History105 BEnrolledStudentsPRIMARY KeyFOREIGN KeyPrimary Keys• A set of fields is a superkey if:– No two distinct tuples can have same values in all key fields• A set of fields is a key for a relation if :– It is a superkey– No subset of the fields is a superkey• what if >1 key for a relation?– One of the keys is chosen (by DBA) to be the primary key.Other keys are called candidate keys.• E.g.– sid is a key for Students.– What about name?– The set {sid, gpa} is a superkey.Primary and Candidate Keys in SQL• Possibly many candidate keys (specified usingUNIQUE), one of which is chosen as the primary key.• Keys must be used carefully!• “For a given student and course, there is a single grade.”“Students can take only one course, and no two studentsin a course receive the same grade.”CREATE TABLE Enrolled (sid CHAR(20) cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid,cid))CREATE TABLE Enrolled (sid CHAR(20) cid CHAR(20), grade CHAR(2), PRIMARY KEY (sid), UNIQUE (cid, grade)) vs. Foreign Keys, Referential Integrity•Foreign key: Set of fields in one relationthat is used to `refer’ to a tuple in anotherrelation.– Must correspond to the primary key of the otherrelation.– Like a `logical pointer’.• If all foreign key constraints are enforced,referential integrity is achieved (i.e., nodangling references.)Foreign Keys in SQL• E.g. Only students listed in the Students relationshould be allowed to enroll for courses.– sid is a foreign key referring to Students: CREATE TABLE Enrolled (sid CHAR(20),cid CHAR(20),grade CHAR(2), PRIMARY KEY (sid,cid), FOREIGN KEY (sid) REFERENCES Students )sid cid grade53666 Carnatic101 C53666 Reggae203 B53650 Topology112 A53666 History105 BEnrolledsid name login age gpa53666 Jones jones@cs 18 3.453688 Smith smith@eecs 18 3.253650 Smith smith@math 19 3.8Students11111 English102 A4Enforcing Referential Integrity• Consider Students and Enrolled; sid in Enrolled is aforeign key that references Students.• What should be done if an Enrolled tuple with a non-existent student id is inserted? (Reject it!)• What should be done if a Students tuple is deleted?– Also delete all Enrolled tuples that refer to it?– Disallow deletion of a Students tuple that is referred to?– Set sid in Enrolled tuples that refer to it to a default sid?– (In SQL, also: Set sid in Enrolled tuples that refer to it to aspecial value null, denoting `unknown’ or `inapplicable’.)• Similar issues arise if primary key of Students tuple isupdated.Integrity Constraints (ICs)• IC: condition that must be true for anyinstance of the database; e.g.,
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