The Entity-Relationship Model, P. I R. Nakatsu

Data Modeling A data model is the relatively simple representation, usually graphic, of the structure of the database. The database designer usually employs data models as communication tools to facilitate interaction among the designer, the applications programmer, and the end user.

Entity-Relationship (ER) Modeling The ER model serves as the basic database blueprint. Entity: anything about which data are to be collected and stored (corresponds to a table; represented by a rectangle) Attribute: a characteristic of an entity (corresponds to a column in a table) Relationship: describes an association between entities (represented by a line joining the two entities together).

Database Systems, 9th Edition

Entities Something that users want to track. © 2000 Prentice Hall

Attributes (properties) Describe the entity’s characteristics. Entity:Employee Attributes:LastName, FirstName, DateOfHire, DateofBirth

Figure 4.3 – Depicting Attributes on an ERM 7

Types of Attributes Required vs. Optional Simple vs. Composite Single-valued vs. Multi-valued Derived (or Calculated) Identifier Provide examples of each type.

Advantages and Disadvantages of Storing Derived Attributes 9

Identifier An attribute or attributes that name or identify entity instances. Note: Can either be unique or nonunique Entity: Employee Identifier 1: SocialSecurityNumber Identifier 2: LastName, FirstName

Relationship Types One-to-One e.g., one dean to one university one head-of-state to one country One-to-Many e.g., one painter paints many paintings one customer places many orders Many-to-Many e.g., students enroll in courses

Determining the Relationship type: 1:1, 1:M, M:N Two questions to identify the relationship type between A and B: –How many instances of B are related to one instance of A? –How many instances of A are related to one instance of B?

Cardinality in an ER Diagram Cardinality expresses the minimum and maximum (min, max) number of entity occurrences associated with one occurrence of the related entity.

Expressing Minimum Cardinality: Hash Marks and Ovals Hash mark (or vertical line): indicates a mandatory relationship (min cardinality of 1) Circle: indicates an optional relationship (min cardinality of 0)

Expressing Optional/Mandatory Relationships On ER Diagram

ER Diagram Problem 1 Acme Insurance Company employs hundreds of claims adjustors across the U.S. As part of its agreement when joining the company, each adjustor is assigned to a company automobile. A claims adjustor works on a number of insurance claims. Sometimes a team of adjustors (two or more adjustors) will work together on a single claim, and there is always at least one adjustor assigned to a claim. Draw the ER Diagram for this situation. Include relationship types using crow’s foot notation, cardinalities in parentheses notation, and hash marks / circles to indicate optional/mandatory relationships.

Existence Dependence 17 Existent DependentExistent Independent Entity exists in the database only when it is associated with another related entity occurrence. Entity exists apart from all of its related entities Referred to as a strong entity or regular entity.

Weak Entity Conditions –Existence-dependent –Has a primary key that is partially or totally derived from parent entity in the relationship –An entity that is not weak is called a strong entity. 18

Weak Entity Examples Example 1: Building (BuildingName) Apartment (BuildingName, Apartment#) Example 2: Employee (EmployeeNo) Dependent (EmployeeNo, DependentNo)

Two Types of Relationships HAS-A relationships: The term is used because an entity has a relationship with another entity (1:1, 1:M, M:N) IS-A relationships: This term refers to relationships between super-types and sub-types.

Entity Supertypes and Subtypes Entity supertype –Generic entity type related to one or more entity subtypes –Contains common characteristics Entity subtype –Contains unique characteristics of each entity subtype

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Disjoint and Overlapping Subtypes Disjoint subtypes (denoted by d) –Each entity instance of the supertype can appear in only one of the subtypes. Overlapping subtypes (denoted by o) –Each entity instance of the supertype can appear in more than one subtype.

Completeness Constraint Specifies whether a supertype occurrence must be a member of at least one subtype Partial completeness –Symbolized by a circle over a single line –Some supertype occurrences are not members of any subtype Total completeness –Symbolized by a circle over a double line –Every supertype occurrence must be a member of at least one subtype

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ER Diagram Problem 2 ABC University is made up of several departments. Each department employs several professors, and each professor is always assigned to a single department. A rule in the university stipulates that a department must consist of between 4 and 20 professors, inclusive. Each department is chaired by a department chair, who is a professor. Draw the ER Diagram for this situation.