Problem. Carnegie Mellon Univ. Dept. of Computer Science /615 - DB Applications

Transcription

1 Carnegie ellon Univ. Dept. of Computer Science /615 - DB Applications C. Faloutsos - A. Pavlo Lecture#2: E-R diagrams Problem Develop an application for U.G. admin: Student info Who--what class Class rosters Transcripts How do you proceed? (Which role(s) are you playing?) 2 Database Design Database Design Requirements Analysis Conceptual Design Logical Design Schema Refinement Physical Design Security Design Requirements Analysis Conceptual Design Logical Design Schema Refinement Physical Design Security Design user s needs high level (ER) Tables ormalization Indices etc Access controls 3 4 1

2 aintain Problem This or this? Develop an application for U.G. admin: Student info Who--what class Class rosters Transcripts If you are the *new* DBA, what would you rather inherit: drop table if exists student; create table student ( fixed, name char(20)); drop table if exists ; create table ( fixed, cid char(10), grade fixed); Student Takes Course 5 6 True story Health insurance company Wants to catch (some of the abundant) fraud Schema: patients, visit doctors, get medicine, Doctors perform operations, urses monitor patients, etc etc Q: How many tables do you think it spans? True story Schema: patients, visit doctors, get medicine, Doctors perform operations, urses monitor patients, etc etc Q: How many tables do you think it spans? 10? 20? 30? 7 8 2

3 True story Schema: patients, visit doctors, get medicine, Doctors perform operations, urses monitor patients, etc etc Q: How many tables do you think it spans? 10? 20? 30? A: 120 PAGES of schema 9 otivation & upcoming conclusion: E-R diagrams are excellent documentation tools Student Takes Course 10 Overview Tools Student Takes concepts Entities Entities ( entity sets ) Course Relationships Attributes Specialization/Generalization Aggregation ER modeling questions P Relationships ( rel. sets ) and mapping constraints attributes

4 Example Students, taking courses, offered by instructors; a course may have multiple sections; one instructor per section nouns -> entity sets verbs -> relationship sets Example Students, taking courses, offered by instructors; a course may have multiple sections; one instructor per section nouns -> entity sets verbs -> relationship sets Example Students, taking courses, offered by instructors; a course may have multiple sections; one instructor per section name... STUDET nouns -> entity sets verbs -> relationship sets 15 ISTRUCTOR i primary key = unique identifier -> underline 16 4

5 name... STUDET STUDET c-id COURSE c-name s-id SECTIO c-id COURSE i ISTRUCTOR but: sections of course (with different instructors)? i ISTRUCTOR but: s-id is not unique... (see later) STUDET STUDET c-id c-id s-id SECTIO COURSE s-id SECTIO COURSE i ISTRUCTOR Q: how to record that students take courses? i ISTRUCTOR

6 STUDET STUDET c-id c-id s-id SECTIO COURSE s-id SECTIO 1 has COURSE teaches 1 teaches 1 ISTRUCTOR ISTRUCTOR to 1 (example?) 1 to to Cardinalities Cardinalities COUTRY 1 1 has 1 PERSO owns CAPITAL CAR STUDET SECTIO

7 Cardinalities Cardinalities COUTRY has CAPITAL Book s notation COUTRY 1 1 has CAPITAL Book s notation: PERSO owns CAR vs 1 to notation PERSO 1 owns CAR STUDET SECTIO STUDET SECTIO Cardinalities COUTRY 1 1 has CAPITAL Total/partial participation 1:1 1:1 total, total COUTRY has CAPITAL PERSO 1 owns CAR PERSO?:1?: owns CAR STUDET SECTIO STUDET?:?: SECTIO

8 Total/partial participation 1:1 1:1 total, total COUTRY has CAPITAL Total/partial participation 1:1 1:1 total, total COUTRY has CAPITAL partial, total PERSO 1:1 owns 0: CAR partial, total PERSO 1:1 owns 0: CAR STUDET?:?: SECTIO partial, total STUDET 1: 0: SECTIO Total/partial participation Total/partial participation Is it legal? partial, total PERSO 1:1 0: owns CAR Is it legal? O! why not? partial, total PERSO 1:1 0: owns CAR

9 Subtle concept: Weak entities section has no unique-id of its own!(?) c-id Weak entities weak entities: if they need to borrow a unique id from a strong entity - thick box. c-id + s-id : unique id for SECTIO partial key (eg., s-id ) - dashed-underline identifying relationship (eg., has ) c-id s-id SECTIO 1 has COURSE s-id SECTIO 1 has COURSE Weak entities Other example(s) of weak entities? Weak entities Other example(s) of weak entities? id e-id 1?? name Dependent 1 has Employee

10 ore details ore details self-relationships - example? self-relationships - example? 1 manages EPLOYEE ore details self-relationships - example? ore details 3-way and k-way relationships? Has-friend FB user

11 ore details 3-way and k-way relationships? Rare, but possible: EPLOYEE uses TOOL ore details 3-way and k-way relationships? Rare, but possible: PROJECT P Other cases? P ore details 3-way and k-way relationships? Rare, but possible: user reviews P App-store/amazon app reviews keyword concepts Entities Overview Relationships Attributes Specialization/Generalization Aggregation ER modeling questions

12 ore details - attributes key (or primary key): unique identifier underlined, in the ER diagram [not in textbook - FYI: multivalued or set-valued attributes (eg., dependents for EPLOYEE) derived attributes (eg., 15% tip) ] concepts Entities Overview Relationships Attributes Specialization/Generalization Aggregation ER modeling questions Basic Advanced/ rare Specialization Observations eg., students: part time (#credithours) and full time (major) name STUDET IS-A Generalization: exact reverse of specialization attribute inheritance could have many levels of an IS-A hierarchy major FT-STUDET PT-STUDET #credits

13 ore details ore details Overlap constraints Covering constraints B A C Overlap constraints can an entity belong to both B and C? Covering constraints can an A entity belong to neither B nor C? B A C ore details ore details A Overlap constraints - examples? o overlap A with overlap B A C Covering constraints - examples? Total coverage A C B B A C B C B C Partial coverage A B C

14 concepts Entities Overview Relationships Attributes Specialization/Generalization Aggregation ER modeling questions Aggregation computer model (w/ CPU and HD) and aker (eg., Dell, HP) CPU HD? AKER Aggregation treat a relationship as an entity used to express a relationship among relationships CPU HD AKER concepts Entities Overview Relationships Attributes Specialization/Generalization Aggregation ER modeling questions

15 Conceptual design Entity vs attribute Entity vs relationship Binary or ternary relationships? Aggregation? Entity vs. attribute Entity EPLOYEE (w/ emp#, name, job_code,...) Q: How about spouse - entity or attribute? Q: How about dependents? Entity vs. attribute Entity vs. Relationship Entity EPLOYEE (w/ emp#, name, job_code,...) Q: How about spouse - entity or attribute? A: probably, attribute is enough Q: How about dependents? A: Entity - we may have many dependents STUDET SECTIO OR STUDET 1 TAKES 1 SECTIO

16 Binary vs Ternary Relationships usually, binary relationships are cleaner : Binary vs. Ternary Relationships If each policy is owned by just 1 employee: name Employees lot policyid Covers Policies cost pname age Dependents Binary vs. Ternary Relationships name lot pname age Binary vs. Ternary Relationships name lot pname age If each policy is owned by just 1 employee: Employees Bad design Covers Policies policyid cost Dependents If each policy is owned by just 1 employee: Key constraint on Policies would mean policy can only cover 1 dependent! Employees Bad design Covers Policies policyid cost Dependents

17 Binary vs. Ternary Relationships If each policy is owned by just 1 employee: Key constraint on Policies would mean policy can only cover 1 dependent! What are the additional constraints in the 2nd diagram? name Employees Bad design name Employees lot policyid Purchaser Better design Covers Policies cost pname pname Beneficiary Policies 65 policyid cost lot age Dependents age Dependents Binary vs Ternary Rel. But sometimes ternary rel. can not be replaced by a set of binary rel s: 66 Binary vs. Ternary Relationships (Contd.) qty Binary vs. Ternary Relationships (Contd.) qty Parts Contract Departments Parts Contract Departments Suppliers VS. Suppliers VS. Parts Departments Parts Departments can-supply Suppliers deals-with can-supply Suppliers deals-with why is it bad? 67 S can-supply P, D needs P, and D deals-with S does not imply that D has agreed to buy P from S. How do we record qty? 68 17

18 Binary vs. Ternary Relationships (Contd.) Binary vs. Ternary Relationships (Contd.) qty Parts Contract Departments ot in textbook: in practice, often: ot in textbook: in practice, often: Suppliers qty qty c-id Parts Contract Departments Parts Contract Departments Suppliers Suppliers Binary vs. Ternary Relationships (Contd.) Ternary vs. aggregation ot in textbook: in practice, often: use aggregation, if we want to attach a relationship to a relationship Parts qty Contract c-id Departments (see book for example) (in practice, again we create a unique-id and resort to binary relationships) 1 1 Suppliers

19 Ternary vs. aggregation How would you handle this case? Ternary vs. aggregation How would you handle this case? COP. ODEL CPU HD AKER CPU HD AKER Ternary vs. aggregation How would you handle this case? Ternary vs. aggregation How would you handle this case? COP. ODEL COP. ODEL CPU???? HD AKER HAS_CPU CPU HD AKER

20 Summary E-R Diagrams: a powerful, user-friendly tool for data modeling: Entities (strong, weak) Attributes (primary keys, discriminators, derived, multivalued) Relationships (1:1, 1:, :; multi-way) Generalization/Specialization; Aggregation POPULAR Summary E-R Diagrams: a powerful, user-friendly tool for data modeling: Entities (strong, weak) Attributes (primary keys, discriminators, derived, multivalued) Relationships (1:1, 1:, :; multi-way) Generalization/Specialization; Aggregation Summary - cont d Summary - cont d (strong) entity set attribute cardinalities cardinalities weak entity set A primary key relationship set A partial key partial/total l:h l :h cardinalities with limits identifying rel. set for weak entity (not in textbook - FYI)

21 Summary - cont d IS-A aggregation 81 21