25SpER Model YenChuPan

8/8/2019 25SpER Model YenChuPan

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Entity-Relationship Data

Model

CS 157A

Professor Sin-Min Lee

Student: Yen-Chu Pan


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Elements of E-R Model In the E/R model, the structure of data is

represented graphically, as an entity-

relationship diagram, using three Principalelement types: Entity Sets.

An entity is an abstract object of some sort, and acollection of similar entities forms an entity set.

Attributes The properties of the entities in the set.

Relationship The connections among two or more entity Sets


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Example of Elements of E-R

Model Entity Sets Departments

Professors

Students Administrators

Attributes Name of Departments, Phone No., Address...

Name, SSN, Address of Professors...

Relationship Students and Professors are under a certain

department

Admin manage the campus/ departments


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Example of the 3 elements in

E/R Diagram


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Classification of Constraints

1. Keys

2. Single-value constraints

3. Multi-valued constraints

4. Mapping Cardinalities and

Participation Constraints


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Key in the E/R Model

Superkeyis a set of one or more attributes that, takencollectively, for us to identify uniquely an item in the entity set.For example, customer-id is a superkey.

Candidate keyis a minimal superkey. For example, customer-name and customer-street is sufficient to distinguish amongmembers of the customer entity set. Then {customer-name,customer-street } is a candidate key.

Primary keydenotes a candidate key that is chosen by the

database designer as the principal means of identifying itemswithin an entity set. the primary key should be chosen suchthat its attributes are never, or very rarely, changed. Forexample, Social-security numbers are guaranteed to neverchanged.


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Single/M

ulti-valued attributes Single-valued attributes are attributes that

only have a single value for a particular

entity. Multi-valued attributes refers to items that

are not singled-value and Null valued. Forexample, consider an employee entity set

with the attribute phone-number. Anemployee may have zero, one, or severalphone numbers; different employee mayhave different numbers of phones.


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Mapping Cardinalities or

Cardinality ratios Express the number of items to which another item

can be associated via a relationship set

Are most useful in describing binary relationshipsets. For a binary relationship set R between entitysets A and B, the mapping cardinality must be oneof the following: One to One

One to Many

Many to One

Many to Many


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Participation Constraints

The participation of an entity set E in a

relationship set R is said to be total, if

every item in E participates in at least

one relationship in R. If only some

items in E participate in relationship R,

the participation of entity set E inrelationship R is said to bepartial.


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Weak Entity Sets

There is an occasional condition in

which an entity sets key is composed

of attributes some or all of which

belong to another entity set. Such an

entity set is called a weak entity set.


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Discriminator The discriminator of a weak entity setis a

set of attributes that allows this distinction to

be made. For example, the discriminator ofa weak entity set payment is the attribute

payment-number, since, for each loan a

payment number uniquely identifies one

single payment for that loan. Thediscriminator of a weak entity set is also

called the partial key of the entity set.


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Requirements forWeak Entity

Sets We cannot obtain key attributes for a weak

entity set indiscriminately. Rather, if E is a

weak entity set then its key consists of: Zero or more of its own attributes, and

Key attributes from entity sets that are reached

by certain many-one relationship from E to other

entity sets. These many-one relationship arecalled supporting relationships for E.


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Discriminator (cont.) Note: although each payment entity is

distinct, payments for different loans may

share the same payment-number. Thus,payment entity set does not have a primary

key; it is a weak entity set.

The primary key of a weak entity set is

formed by the primary key of the identifyingentity set, plus the weak entity sets

discriminator.


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Identifying aW

eak Entity Type A Weak entity type doesnt have a primary key.

If X is a weak entity type and Y is the entity typeon which X is dependent.

We form a primary key for X by combining theprimary key of Y which one or more attributes,called discriminatororpartial key, from X.

In an E/R Diagram, a partial key is usually dash-

underlined. e.g., primary key for DEPENDENT: {Employee No.,

DName}.


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* Doted-line = double-line

*


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References Peter Chens website:

http://bit.csc.lsu.edu/~chen/chen.html

Database Systems: A First Course, J.D.Ullman & J. Widom

http://www-db.stanford.edu/~ullman/fcdb.html

http://www.cs.sjsu.edu/faculty/lee/cs157/25S

p157AL4.ppt http://www.cs.sjsu.edu/faculty/lee/cs157/25S

p157AL5Enhanced%20ER-diagram.ppt

25SpER Model YenChuPan

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