MIS 385MBA 664 Systems Implementation with DBMS Database

MIS 385/MBA 664 Systems Implementation with DBMS/ Database Management Dave Salisbury salisbury@udayton. edu (email) http: //www. davesalisbury. com/ (web site)

Modeling reality n n 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 n n n 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: n n n n 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) n n Representation of structure and constraints of database independent of software Mainstream approach to conceptual modeling is ERD n n n 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 n Entity instance - person, place, object, event, concept (often corresponds to a row in a table) n n n 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) n Relationship type – category of relationship…link between entity types

Elements of an E-R diagram ENTITY ATTRIBUTE RELATIONSHIP WEAK ENTITY MULTIVALUED ATTRIBUTE INDENTIFYING RELATIONSHIP ASSOCIATIVE ENTITY DERIVED ATTRIBUTE

What should an entity be? n Should be: n n 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: n n A user of the database system An output of the database system (e. g. a report)

Attributes n n 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 n Customer_Number n n Last_Name n n Zip n n Xenia State n n 2425 Jasper Road City n n Dave Address n n Salisbury First_Name n n 12345 OH 45385 Phone n 937 -293 -0258

Identifiers (keys) n n n 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 n n 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

Simple key attribute The key is underlined

Composite Key The key is composed of two subparts

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

Both multivalued & composite attribute This is an example of timestamping

More on relationships n Relationship Types vs. Relationship Instances n n Relationships can have attributes n n n The relationship type is modeled as the diamond and lines between entity types…the instance is between specific entity instances 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 n More on this later

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

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

Relationship cardinality notation Mandatory one Mandatory many Optional one Optional many

Unary

Binary

Ternary

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

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

Strong vs. weak entities & identifying relationships n Strong entities n n Weak entity n n exist independently of other types of entities has its own unique identifier represented with single-line rectangle dependent on a strong entity…cannot exist on its own Does not have a unique identifier represented with double-line rectangle Identifying relationship n n links strong entities to weak entities represented with double line diamond

Strong & weak entities Weak entity Strong entity Identifying relationship

Associative entities n n n 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? n n n 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 112 -113)

Maximum & minimum cardinalities Only maximum showing Mandatory minimums showing

Optional cardinality

Relationship type; entity & relationship instances

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

Unary with an attribute Representing a bill-of -materials structure

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

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

Multivalued attribute vs. relationship

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

Ternary as Associative Entity

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

Pine Valley Furniture

Notation used in earlier edition of our textbook – what’s different?

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

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

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

Access Notation