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Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints...

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Lecture 2 Entity-Relationship Model Eugene Wu
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Page 1: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Lecture 2Entity-Relationship Model

Eugene Wu

Page 2: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Steps for a New Application

Requirementswhat are you going to build?

Conceptual Database Designpen-and-pencil description

Logical Designformal database schema

Schema Refinement:fix potential problems, normalization

Physical Database Designuse sample of queries to optimize for speed/storage

App/Security Designprevent security problems

Page 3: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Steps for a New Application

Requirementswhat are you going to build?

Conceptual Database Designpen-and-pencil description

Logical Designformal database schema

Schema Refinement:fix potential problems, normalization

Physical Database Designuse sample of queries to optimize for speed/storage

App/Security Designprevent security problems

ERModeling

Page 4: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Database Apps Are Complicated

Typical Fortune 100 Company~10k different information (data) systems

90% relational databases (DBMSes)

Typical database has >100 tables

Typical table has 50 – 200 attributes

Page 5: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

How Hard? Wikimedia

https://upload.wikimedia.org/wikipedia/commons/f/f7/MediaWiki_1.24.1_database_schema.svg

Page 6: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

How Hard? Wikimedia

https://upload.wikimedia.org/wikipedia/commons/f/f7/MediaWiki_1.24.1_database_schema.svg

Page 7: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Inconsistencies/Constraint Violations

Huge amount of effort to avoid inconsistenciesCan data model help us avoid automatically?

DBLP is the site for computer science publications

Page 8: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Inconsistencies/Constraint Violations

≠

Page 9: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Inconsistencies/Constraint Violations

Check in application code!

Page 10: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

It is Hard to Design Applications

DBMS

Server Code

Page 11: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

It is Hard to Design Applications

DBMS

Server CodeAdd new users

Page 12: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

It is Hard to Design Applications

DBMS

Server CodeAdd new users

Normal Signup

Developeraccount

App client App client

Auto added

App client

Page 13: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

It is Hard to Design Applications

DBMS

Server Code

App client App client App client

check_unique(username)

Normal Signup

Developeraccount

Auto added

Page 14: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Let’s make a webapp $$$

live exercise time

Page 15: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,
Page 16: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Entity-Relationship Modeling

Entities (objects) to store and their attributes

Relationships between entities and their attrs.

Integrity constraints & business rules

Visually modeled, easy to turn into DB schema

CoursesCourse NumberCourse TitleYearSemester

Page 17: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

UsersNicknameNameBirthdaySummaryEmail…

Page 18: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Entity e.g., intro to databasesreal-world object distinguishable from other objectsdescribed as set of attributes & the values(think one record)

Entity Set e.g., all coursescollection of similar entitiesall entities have same attributes (unless Is-A)must have one or more keysattributes have domains≈ table

Basics: Entities

Page 19: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Course

cidloc

schedule

Example: Entity

Keys (cid, uid) are underlinedValues must be unique

(think: can use as hashtable key to lookup in table)

name

Users

uidage

summaryname

Page 20: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Basics: Relationships

Relationship: association between 2 or more entitiese.g., alice is taking Introduction to DBs

Relationship Set: collection of similar relationshipsN-ary relationship set R relates N entity sets E1 … En

Each r∈R involves entities e1 … en

An Ei can be part of diff. relationship sets or diff. roles in same set

Page 21: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Basics: Relationships

Takes

Courses

Users

Users

Teacher_of Student_of

uid agename

Teaches

Users takes diff roles in same relationships set

Page 22: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Basics: Relationships

Relationships sets can have descriptive attributese.g., the since attribute of Instructs

Takessince

Courses

Users

Page 23: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Basics: Ternary Relationships

Connects three entitiesN-ary relationships possible too.

GradedGrade

Courses

Users

Assignments

Page 24: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Constraints

Help avoid corruption, inconsistencies

Key constraints

Participation constraints

Weak entities

Overlap and covering constraints

Page 25: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Key Constraints

Defines cardinality requirements on relationships

Many to many e.g., consider Takes

a user can take many courses

a course can have many users that take the courseOne to Many e.g., consider Instructs

a course has at most one instructor

Many-to-Many1-to-1 1-to Many

Instructs

Users

Courses

Page 26: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Instructs

Users

Courses

A course has at most one instructor

Instructs

Users

Courses

???

Page 27: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Participation Constraints

Does every course need an instructor?

If yes, it’s a participation constraint

e.g., participation of Courses in instructs is Total

Otherwise, partial participation constraint

Users Courses

Instructs

Takes

since

Page 28: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Weak Entities

A weak entity can only be uniquely identified by using the primary key of its owner entity

Owner and weak entity sets must be in one to many relationship set

Weak entity set must have total participation in this identifying relationships set

Posted

Users

WallPosts

time

Page 29: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

At most one

At least one

Exactly one

Weak Entity

Page 30: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

ISA (is a) Hierarchies

Inheritance rules similar to programming languagesA ISA B à every A also considered a BWhen querying for Bs, must consider As (unlike e.g., C++)

Why use ISA?add descriptive attributes specific to a subclass e.g., grade

identify entities that participate in a relationship

ISA

Instructors (A) Students (A)Rating Grade

Users (B)

Page 31: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

ISA (is a) Hierarchies

Overlap Constraint can eugene be an instructor and a student? (allow/disallow)

Covering Constraintmust every user be an instructor or student? (yes/no)

ISA

Instructors (A) Students (A)Rating Grade

Users (B)

HOW DO WE EXPRESS THESE IN AN ER DIAGRAM???????

Page 32: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Manages

Instructors

Since

Aggregation

Relationships between (entities – relationships)

Lets us treat a Relationship Set like an Entity Setso it can participate in other relationships

Companies Courses

Amount

Donates

Page 33: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Aggregation vs Ternary Relationships

Why use aggregation?Manages and Donates are distinct relationships with own attrsCan define constraints on relationship sets

e.g., a donation can be managed by at most one instructor

Manages/DonatesCompanies

Instructors

Courses

AmountSince

Page 34: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Aggregation vs Ternary Relationships

Constraints apply to all connected entity sets

Manages/DonatesCompanies

Instructors

Courses

AmountSince

Page 35: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Using the ER Model

Design Choices for a concept

Entity or Attribute?

Entity or Relationship?

Binary or Ternary relationship?Aggregation or Ternary relationship?

Page 36: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Entity or Attribute?

Is users.address an attribute of Users or an entity connected to Users by a relationship?

Depends (and may change over time!)If a user has >1 addresses, must be an entity

If an address has attrs (structure), must be entity

e.g., want to search for users by city, state, or zip

Page 37: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Entity or Attribute?

A company can’t donate multiple amounts (top fig)

Use ternary relationship (bottom fig)

Amount

DonatesCompanies Courses

Donates

Donations

Companies Courses

AmountWhen

Page 38: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Entity or Relationship?

OK if company donates to courses individually

What if company donates to school for all data-related courses?Redundancy of amount, need to remember to update every one

Misleading implies amount tied to each donation individually

Company Course AmountAmazon 4111 2000Amazon 4112 2000Amazon 5111 2000

These amounts are logically the same (redundant)!

Amount

DonatesCompanies Courses

Page 39: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Entity or Relationship?

If company donates once to school for data related courses.

Refactor amount into an entity

Company Course DonationAmazon 4111 1Amazon 4112 1Amazon 5111 1

Donation When Amount

1 Today 2000

Donates

Donations

Companies Courses

AmountWhen

Page 40: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Binary or Ternary Relationship?

What if assignments have at most one grader?

EvaluatesGrade

Grader

Assignment

Student

Page 41: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Binary or Ternary Relationship?

What if assignments have at most one grader?Only one student can complete HW0!

Actually two separate relationships

EvaluatesGrade

Grader

Assignment

Student

Page 42: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Binary or Ternary Relationship?

Binary relationships allows additional constraints

Graders

Assignment

Students

Graded Submitted

What should happen if a student drops the class? (see next slide)

Page 43: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Binary or Ternary Relationship?

Binary relationships allows additional constraints

Graders

Assignment

Students

Graded Submitted

What should happen if a student drops the class? The previous ER diagram is sufficient.This is wrong! You don’t need a weak entity to express this.In fact, this would mean that if Jane completed HW0 and then dropped the class, then the entire HW0 (not just her submission) would disappear.

Page 44: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Binary or Ternary Relationship?

Sometimes have true ternary relationship that is defined by all three entities.

qty

Bookstores

Books

Customer

Buys

Page 45: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Binary or Ternary Relationship?

Sometimes have true ternary relationship that is defined by all three entities.

Bookstores

Books

Customer

Sells

Buys

Buys

Doesn’t Really Work

Page 46: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Using ER Modeling

Constraints in ER Modeling

Many types of data semantics can be captured using ER

Some constraints not captured (discuss limitations later)

Need further schema refinementER Model is still subjective, need further refinement after translated into relational schema

Page 47: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Summary

Requirementswhat are you going to build?

Conceptual Database Designpen-and-pencil description

Logical Designformal database schema

Schema Refinement:fix potential problems, normalization

Physical Database Designuse sample of queries to optimize for speed/storage

App/Security Designprevent security problems

(Today)ERModeling

Page 48: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Summary

Conceptual design follows requirements analysis

ER model helpful for conceptual designconstraints are expressivematches how we often think about applications

Core constructsentity, relationship, attributeweak entities, ISA, aggregation

Many variations beyond today’s discussion

Page 49: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Summary

ER design is subjective based on usage+needsToday we saw multiple ways to model same idea

ER design is not complete/perfectDeveloped in an enterprise-oriented world (ER First)Doesn’t capture semantics (what does “instructor” mean?)

Doesn’t capture e.g., processes/state machinesHow to combine multiple ER models automatically?Limitation of imagination when designing applicationOpen problems!

ER design is a useful way to think

Page 50: Lecture 2 Entity-Relationship Model - GitHub PagesER model helpful for conceptual design constraints are expressive matches how we often think about applications Core constructs entity,

Next Time

Relational Model: de-facto DBMS standard

Set up for ER diagrams à Relational models


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