CSCI 6442Entity-Relation Data Modeling

Copyright 2012, David C. Roberts, all rights reserved

Agenda

•Data Modeling•Natural and Generated Keys

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Relational Database

Remember that a relational database is a collection of time-varying, independent relations

Each relation models some entity type in the problem space (outside the computer)

There is a close correspondence between entity types and relations

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Data Modeling

•We can say that the relational database models the real-world problem

•So construction of a relational database becomes the selection of the entity types to put into the data model

•Sometimes entity type selection is simple; other times entity types are not apparent and selection can be difficult

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Notation for Data Models• Because a relational database is comprised of

independent tables, how do you deal with relationships during design?

• Chen developed the entity-relation data model, which considered entity types and their relationships

• He expected that a new type of database system would evolve, directly implementing the E-R data model

• Instead, the E-R notation was found to be an excellent tool for relational database design and the relational approach has taken over

• Since then, popular automated tools have tended to change the notation for easier computer printing

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Chen’s Notation• We will use Chen’s notation here for two

reasons:1. It separates the notion of a relationship from

the notion of an entity type2. Conversion of the design into relational

tables occurs at the end of the design process, so the difference between the data model and physical design is clear

• In your own work, once you are familiar with the concepts, you will be able to use the popular “crow’s foot” notation used by DBDesigner4

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Entities

•Strong Entity—has independent existence

•Weak Entity—exists only when the entity it depends on exists

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Employee

Dependent

Question: What are someExamples of strong entitytypes? Weak entity types?

Examples of Entity Instances

An instance of an entity is a specific occurrence of an entity type:

•Bill Gates is an Employee of Microsoft•Spam is a Product•Greenpeace is an Organization•Flour is an ingredient

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Attributes

•Attributes are indicated inside ovals; identifier attributes are underlined. For complex diagrams, attributes are often omitted

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Employee

EMPNO

JOB

SALARYQuestion: Which attributes in this diagram should be underlined?

Examples of AttributesAn attribute is a characteristic of an entity type:EmployeeIDSocial Security NumberFirst NameLast NameStreet AddressCityStateZipCodeDate Hired Health Benefits Plan

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Multi-Valued Attribute

•A multi-valued attribute is shown as a double line

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Employee

EMPNO

JOB

SALARY

TELEPHONE

Question: Which attribute should have a double underline?

Relationship

A relationship is an association between two entity types, for example:▫A CUSTOMER places a CUSTOMER

ORDER▫An EMPLOYEE takes a CUSTOMER

ORDER▫A STUDENT enrolls in a COURSE▫A COURSE is taught by a FACULTY

MEMBER•Some say that entity types should be

nouns and relationships should be verbs

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Relationship

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EMPLOYEE

DEPARTMENT

WORKS FOR

Categorizing Relationships•Number of entity types participating:

▫Unary: one▫Binary: two▫Ternary: three

•Existence of related instances(a.k.a. optionality):▫Mandatory▫Optional

•Cardinality of the relationship:▫One-many▫Many-many▫One-one

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One-to-One Relationships• Usually the only time a one-to-one relationship is used is

for a dependent entity. Otherwise, usually if there is a one-to-one relationship, careful consideration will show that there is just one entity type.

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EMPLOYEE

SPOUSE

HAS

1

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Relationship And Cardinality

•A relationship is an association between two or more entity types, drawn as a diamond. Relationships may be one-to-many, many-to-many or one-to-one

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DEPARTMENT

EMPLOYEE

HAS

STUDENT

COURSE

GRADE

EMPLOYEE

COMPANY CAR

HAS

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N N

N 1

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Recursive Relationship

•Recursive relationship is how a repeated hierarchy is represented

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Employee WORKS FOR

Question: How else can this hierarchy be represented?

Relationship of Higher Degree• A ternary relationship, also said to have degree

3.

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STUDENT

COURSE

REG

N

N

TEACHERN

COURSENO

GRADE

Optionality of Participation

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EMPLOYEE

HAS

STUDENT

COURSE

GRADE

EMPLOYEE

COMPANY CAR

HAS

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N N

N 1

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DEPARTMENT

Supertypes

•It can be useful to consider a supertype that includes several entity types as subtypes

•Supertypes can be▫Complete: every instance of the supertype

is one of the subtypes▫Distinct: no single entity type can be a

member of two subtypes

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Notation

•Chen’s notation for supertypes and subtypes is cumbersome, so let’s use “crow’s-foot” notation

•A supertype is shown as a box surrounding the boxes for all the subtypes

•Relationship lines end on the supertype or the subtype boundaries as appropriate

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Subtypes

•Subtypes can be either mutually exclusive (disjoint) or overlapping (inclusive). ▫For a mutually exclusive category, an entity

instance may be in only one subtype. ▫For an overlapping category, an entity

instance may be in two or more subtypes. •The completeness constraint: all

instances of a subtype must be represented in the supertype.

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Example Subtypes

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PARTY

EMPLOYEE

APPLICANT

PERSON

ORGANIZATION

SUPPLIER

CUSTOMER

Question: How would this be implemented as database tables?PARTY(PARTYID)

PERSON(PARTYID,FNAME,MI,LNAME,DOB,POB)EMP(PARTYID,EMPID,DHIRED,SAL,JOB,DEPTNO)APP(PARTYID,DAPPLIED,STATUS)ORG(PARTYID,ORGNAME,ORGCITY,ORGSTATE,ORGZIP)CUST(PARTYID,CUSTID,DOFO,DOLO,LYVOL)SUPP(PARTYID,SUPPID,DOFO,DOLO,LYVOL)

Why Use Subtypes and Supertypes Like This?

•Relationships are simplified▫The relationship can take place at the

highest possible level in the hierarchy▫Fewer relationship tables are generally

required•Programming can be simplified

▫All subtypes can be processed in the same way

▫For example, write a check to an organization or a check to a supplier with the same code

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Relationships

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DEPT WORKS IN

BELONGS TO

Converting an ERD to Relational•Recall that when we did an ERD we were

not designing tables, we were defining the problem in terms of entity types and relationships

•Now the design must be translated into relational tables

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Question: What are the symbols on the ERD that will become tables in the relational database?

Converting to Relational•Every entity type becomes a table•For one-many relationships, put the

identifier of the one with each of the many•For one-one relationships, put the

identifier of each with the other•For many-many relationships, identify an

entity type that connects them, and make it a table, using the primary key of both partners as a composite primary key

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Conversion

•We see that every rectangle in the ERD will become a table in the relational database

•Some of the diamonds will become tables and some will not

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Students and Courses

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Course

NumberLocation

Credits

Registration

Grade

StudentIDName

Student

Grad YR

What About Subtypes?

•Each subtype and supertype is a separate table

•Common attributes are stored with the highest level entity that shares them

•The root level supertype may have many attributes, or it may have only a key

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OK, How Do I Do It?You will have your own ideas about how to proceed, but here’s an idea:1. Enumerate things in the problem space to keep

track of (these are entity types)2. Enumerate what to keep track of about each

thing (these are attributes)3. Enumerate relationships to keep track of4. Draw a draft ERD and review it for difficulty of

doing the operations that are needed5. Review it for possible simplifications and

alternative approaches

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Here’s Another Approach• List all the facts you want to keep track of and their

subjects• Group your list by subject• Consider each subject as an entity type, the facts

about it as attributes• Be sure to use separate entity types for repeated

groups of facts• Now draw your draft ERD and review it for

difficulty of programming the operations that are needed

• Review it for simplifications and alternate approaches

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Natural and Generated Keys

Definitions

Natural key—a key that occurs in the data, that uniquely identifies rows. AKA candidate key.

Generated key—a key that is generated at the time data is inserted into the database, that did not occur naturally

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Postulate:In a normalized data model, every row has a natural

key

Proof: 3NF requires that the value of every attribute be ffd by the primary key. By construction, primary keys uniquely identify instances and are unique. Therefore, each row has a different value for the primary key and is hence unique.

Result: There is no reason to add a primary key to normalized data in order to make rows unique. They already are.

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Question: does 3NF require that there be a single-attribute primary key?

Generated KeysIn some organizations, programmers have the idea that a

generated key should be included in every relation, a misunderstanding of the relational approach. Often they will say “it’s to make rows unique” or “to provide a unique identifier.” This is terribly wrong!

That’s a fine idea for an Excel spreadsheet but it shows a misunderstanding of what a relation is and what a tuple is.

Tuples are naturally unique because each one corresponds to a distinct instance of an entity type in the real world. You don’t have to add anything to make them unique.

If your database design is correct, you have unique rows even if you haven’t added a key to each row.

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Making the Point Again, Again

•Each row corresponds to an occurrence of a entity instance

•Each entity instance is unique; that’s why it is tracked as an instance

•Therefore, every row is different from every other row without adding anything to it

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Question: what if entity instances are not unique? How do we track them in a database?

Claim That’s Made for Generated Keys

• Some think that generated keys allow more flexibility• For example, in our EMP table, can we allow a

person’s name to change?• If we use a generated key, then the name can change

and all other associated data will still be associated with the correct person

• What do you think of this statement?• Can you use telephone number as the primary key?

Why or why not?• Can primary keys ever change in value?• And what if the primary key is a foreign key in

another table?

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SQL and Primary Keys

•SQL allows primary keys to change; they are just a value that changes

•What about foreign keys? Can they change?

•In the CREATE TABLE statement, you state ON UPDATE or ON DELETE CASCADE, RESTRICT, NO ACTION, SET NULL or SET DEFAULT. For ON DELETE, DELETE can also be an action

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Limitation of SQL Database Systems

•In general, the primary key is not allowed to have null value—which is fine

•But a composite primary key is not allowed to have any part that is null

•The composite primary key limitation does not correspond to reality, can force us to use a generated key

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Benefits of Natural Keys:

•Assures proper attention to the data model, because unique values must be identified

•Tables have fewer columns•Avoid storage of meaningless data•The key value itself may have value to an

application, hence will avoid retrieval of the row itself

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When to Use Generated Keys• Sometimes you have a compound key that is very

long• Lots of queries will be very complicated and tables

with foreign keys will copy most of another table• As a guideline, the only time to use a generated

key is when the natural key has at least three columns

• A special case of this rule is the case where the whole row would have to be a composite key without a generated key

• Example: internal person

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Another Case for Generated Keys•Suppose we don’t have any kind of single

or composite identifier that is always present

•Database systems generally don’t allow for part of a designated composite key to be null

•Even though it’s legitimate, because of database system limitations, you may have to use generated keys if you don’t have a common identifier without any null parts, ever

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Example

•ID Card for entry to GWU buildings•Students may not have SSN but all have

student number•Contractors may not have GWU employee

# but all have SSN

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The Bottom Line

•Don’t use generated keys. Ever.•Unless you must!

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