The Relational ModelReviewSlide 3GlossaryData ModelsWhy Study the Relational Model?Relational Database: DefinitionsEx: Instance of Students RelationSQL - A language for Relational DBsSQL OverviewCreating Relations in SQLTable Creation (continued)Adding and Deleting TuplesKeysPrimary KeysPrimary and Candidate Keys in SQLForeign Keys, Referential IntegrityForeign Keys in SQLEnforcing Referential IntegrityIntegrity Constraints (ICs)Where do ICs Come From?AdministriviaRelational Query LanguagesThe SQL Query LanguageQuerying Multiple RelationsSemantics of a QueryCross-product of Students and Enrolled InstancesRelational Model: SummaryThe Relational ModelCS 186, Fall 2006, Lecture 2R & G, Chap. 3Review•Why use a DBMS? OS provides RAM and diskReview•Why use a DBMS? OS provides RAM and 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 link between user’s view of the world and bits stored in computer•Many models exist•We will concentrate on the 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 at Berkeley•XML features in most relational systems–Can export XML interfaces–Can embed XML inside relational fieldsRelational 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•Can think of a relation as a set of rows or tuples. –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”), 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 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 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•Keys are one form of integrity constraint (IC)sid name login age gpa53666 Jones jones@cs 18 3.453688 Smithsmith@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 using UNIQUE), 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 students in 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 relation that is used to `refer’ to a tuple in another relation. –Must correspond to the primary key of the other relation. –Like a `logical pointer’.•If all foreign key constraints are enforced, referential integrity is achieved (i.e., no dangling references.)Foreign Keys in SQL•E.g. Only students listed in the Students relation should 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 Smithsmith@eecs 18 3.253650 Smith smith@math 19 3.8Students11111 English102 AEnforcing Referential Integrity•Consider Students and Enrolled; sid in Enrolled is a foreign key that references Students.•What should be done if an Enrolled
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