MIS 385 MBA 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 - Hoffer Entity symbols A special entity that is also a relationship Relationship degrees specify number of entity types involved Attribute symbols Relationship cardinalities specify how many of each entity type is allowed

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

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

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

Key attributes The identifier is boldfaced and underlined

Simple key attribute The key is underlined

Composite Key The key is composed of two subparts

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

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

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 text on the 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 types and instances Relationship type Relationship instances

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

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

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

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 n exist independently of other types of entities own unique identifier underlined with single-line 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 n links strong entities to weak entities

Strong & weak entities Identifying relationship Strong entity Weak entity

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)

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

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

Ternary as Associative Entity

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

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

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

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

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

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

Multivalued attribute vs. relationship 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 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