MIS 385/MBA 664 Systems Implementation with DBMS/ Database Management Dave Salisbury ( ) (web site)

Modeling reality A database must mirror the real world if it is to answer questions about the real world Data Modeling is a design technique for capturing reality STUDENT Social_Security_No Name Major

Business rules Statements that define or constrain some aspect of the business Assert business structure Control/influence business behavior Expressed in terms familiar to end users Automated through DBMS software

A good business rule is: Declarative – what, not how Precise – clear, agreed-upon meaning Atomic – one statement Consistent – internally and externally Expressible – structured, natural language Distinct – non-redundant Business-oriented – understood by business people

The conceptual model (ER diagram) Representation of structure and constraints of database independent of software Mainstream approach to conceptual modeling is ERD ease of use CASE support entities and relationships are “natural” No standard notation Building blocks are entities, attributes, relationships, and identifiers

E-R model constructs Entity instance - person, place, object, event, concept (often corresponds to a row in a table) Entity Type – collection of entities (often corresponds to a table) Attribute - property or characteristic of an entity type (often corresponds to a field in a table) Relationship instance – link between entities (corresponds to primary key-foreign key equivalencies in related tables) Relationship type – category of relationship…link between entity types

Relationship degrees specify number of entity types involved Entity symbols A special entity that is also a relationship Relationship symbols Relationship cardinalities specify how many of each entity type is allowed Attribute symbols Elements of an E-R diagram - Hoffer

Elements of an E-R diagram – Chen* ENTITY RELATIONSHIP WEAK ENTITY INDENTIFYING RELATIONSHIP ATTRIBUTE MULTIVALUED ATTRIBUTE ASSOCIATIVE ENTITY DERIVED ATTRIBUTE *Actually somewhat modified Chen.

What should an entity be? Should be: An object that will have many instances in the database An object that will be composed of multiple attributes An object that we are trying to model Should NOT be: A user of the database system An output of the database system (e.g. a report)

Inappropriate entities System user System output Appropriate entities Inappropriate/Appropriate Entities

Attributes A discrete data element Describes an entity (i.e., is a characteristic) Meaningful (for the system being modeled) Attributes are the items of interest to the organization -- the things being stored Customer_Number Last_Name Salisbury First_Name Dave Address 2425 Jasper Road City Xenia State OH Zip Phone

Identifiers (keys) Identifier (Key) - An attribute (or combination of attributes) that uniquely identifies individual instances of an entity type Simple Key versus Composite Key Candidate Key – an attribute that could be a key…satisfies the requirements for being a key

Characteristics of Identifiers Will not change in value Will not be null No intelligent identifiers (e.g. containing locations or people that might change) Substitute new, simple keys for long, composite keys

Composite Attributes An attribute broken into component parts

Composite Attributes

Key attributes The identifier is boldfaced and underlined

The key is underlined Simple key attribute

The key is composed of two subparts Composite Key

Multi-valued & derived attributes Multivalued an employee can have more than one skill Derived from date employed and current date

Both multivalued & composite attribute This is an example of time-stamping

Derived from date employed and current date What’s wrong with this? Multivalued: an employee can have more than one skill Multi-valued & derived attributes

This is an example of time- stamping Multivalued & composite attribute

More on relationships Relationship Types vs. Relationship Instances The relationship type is modeled as the text on the lines between entity types…the instance is between specific entity instances Relationships can have attributes These describe features pertaining to the association between the entities in the relationship Two entities can have more than one type of relationship between them (multiple relationships) Associative Entity = combination of relationship and entity More on this later

Relationship types and instances Relationship type Relationship instances

Relationship degree Entities of three different types related to each other Entities of two different types related to each other One entity related to another of the same entity type

Relationship degree Unary – related to same entity type Binary – two different entity types Ternary – three different entity types

Cardinality Constraints Cardinality Constraints - the number of instances of one entity that can or must be associated with each instance of another entity. Minimum Cardinality If zero, then optional If one or more, then mandatory Maximum Cardinality The maximum number

Relationship cardinality notation Mandatory one Mandatory many Optional one Optional many

Relationship cardinality notation Mandatory one Mandatory many Optional one Optional many

Unary

Binary

Here, the date completed attribute pertains specifically to the employee’s completion of a course…it is an attribute of the relationship Binary relationship with an attribute

Ternary relationship (w/ attributes) (Relationships can have attributes of their own)

Hierarchical Relationships Occur frequently Model as multiple 1:M relationships Firm Division Dept

Strong vs. weak entities & identifying relationships Strong entities exist independently of other types of entities own unique identifier identifier underlined with single-line Weak entity dependent on a strong entity (identifying owner) cannot exist on its own no a unique identifier (only a partial identifier) Partial identifier underlined with double-line Entity box has double line Identifying relationship links strong entities to weak entities

Strong entity Weak entity Identifying relationship Strong & weak entities

Strong entity Weak entity Identifying relationship Strong & weak entities

Associative entities It’s an entity – it has attributes AND it’s a relationship – it links entities together When should a relationship with attributes instead be an associative entity? All relationships for the associative entity should be many The associative entity could have meaning independent of the other entities The associative entity preferably has a unique identifier, and should also have other attributes The associative may be participating in other relationships other than the entities of the associated relationship Ternary relationships should be converted to associative entities (p )

Associative entity is like a relationship with an attribute, but it is also considered to be an entity in its own right. Associative entity

Associative entity involves a rectangle with a diamond inside. Note that the many-to-many cardinality symbols face toward the associative entity and not toward the other entities Associative entity

This could just be a relationship with attributes…it’s a judgment call Associative Entity

This is the Bill of Materials laid out a different way. It could just be a relationship with attributes…it’s a judgment call Associative Entity

Ternary as Associative Entity

A patient must have recorded at least one history, and can have many A patient history is recorded for one and only one patient Mandatory cardinalities

An employee can be assigned to any number of projects, or may not be assigned to any at all A project must be assigned to at least one employee, and may be assigned to many 1 optional many, 1 mandatory many

A person is is married to at most one other person, or may not be married at all Optional Cardinality

Entities can be related to one another in more than one way Multiple relationships (entities can be related to one another in more than one way)

Employees and departments Multiple relationships (entities can be related to one another in more than one way)

Here, min cardinality constraint is 2 Professors & courses (fixed upon constraint)

Here,max cardinality constraint is 4 Professors & courses (fixed upon constraint)

simple composite Multivalued attribute vs. relationship

Sample E-R Diagram (Figure 3-1)

Sample E-R Diagram (Similar to figure 3-1 with different notation)

Pine Valley Furniture

Modified Chen notation used in an earlier edition of our textbook

Different notation CUSTOMER * Customer ID Customer name ORDER * Order ID Order Date

Third style of notation CUSTOMER ORDER SUBMITS 1 M Customer ID Customer name Order ID Order date

Oracle notation CUSTOMER #Customer_ID *Customer Name ORDER #Order_ID *Order Date

Access Notation