Tables and Their Characteristics Logical view of relational database is based on relation Relation thought of as a table Table: two-dimensional structure composed of rows and columns Persistent representation of logical relation Contains group of related entities (entity set)

Keys Each row in a table must be uniquely identifiable Key is one or more attributes that determine other attributes Key’s role is based on determination If you know the value of attribute A, you can determine the value of attribute B Functional dependence Attribute B is functionally dependent on A if all rows in table that agree in value for A also agree in value for B

Keys (cont’d.) Composite key Key attribute Superkey Candidate key Composed of more than one attribute Key attribute Any attribute that is part of a key Superkey Any key that uniquely identifies each row Candidate key A superkey without unnecessary attributes

Keys (cont’d.) Nulls No data entry Not permitted in primary key Should be avoided in other attributes Can represent: An unknown attribute value A known, but missing, attribute value A “not applicable” condition

Keys (cont’d.) Now let's see how you should choose your key(s). First, let's make up a little table to look at: PersonID LastName FirstName D.O.B 1 Smith Robert 01/01/1970 2 Jones 3 01/01/1972 4 Henry 5

Keys (cont’d.) This table has many superkeys: PersonID PersonID + LastName PersonID + FirstName PersonID + DOB PersonID + LastName + FirstName PersonID + LastName + DOB PersonID + FirstName + DOB PersonID + LastName + FirstName + DOB LastName + FirstName + DOB All of these will uniquely identify each record, so each one is a superkey. Of those keys, a key which is comprised of more than one column is a composite key; a key of only one column is a simple key. What ‘s about FirstName , LastName, FirstName+ LastName ?

Keys (cont’d.) A candidate key is a superkey that has no unique subset; it contains no columns that are not necessary to make it unique. This table has 2 candidate keys: PersonID LastName + FirstName + DOB PersonID + LastName  have subset PersonID and LastName, but LastName is not in the superkey. PersonID is in the superkey, so that PersonID+LastName will not be candidate keys. Select one of the candidate key as a primary key

Referential integrity Foreign key (FK) An attribute whose values match primary key values in the related table Referential integrity FK contains a value that refers to an existing valid tuple (row) in another relation

The Entity Relationship Model (ERM) ER model forms the basis of an ER diagram ERD represents conceptual database as viewed by end user ERDs depict database’s main components: Entities Attributes Relationships

Entities Refers to entity set and not to single entity occurrence Corresponds to table and not to row in relational environment In Chen and Crow’s Foot models, entity is represented by rectangle with entity’s name Entity name, a noun, written in capital letters

Attributes Characteristics of entities Chen notation: attributes represented by ovals connected to entity rectangle with a line Each oval contains the name of attribute it represents Crow’s Foot notation: attributes written in attribute box below entity rectangle

Attributes (cont’d.) Composite attribute can be subdivided Simple attribute cannot be subdivided Single-value attribute can have only a single value Multivalued attributes can have many values

Attributes (cont’d.) M:N relationships and multivalued attributes should not be implemented Create several new attributes for each of the original multivalued attributes’ components Create new entity composed of original multivalued attributes’ components Derived attribute: value may be calculated from other attributes Need not be physically stored within database

Relationships Association between entities Participants are entities that participate in a relationship Relationships between entities always operate in both directions Relationship can be classified as 1:M Relationship classification is difficult to establish if only one side of the relationship is known

Connectivity and Cardinality Describes the relationship classification Cardinality Expresses minimum and maximum number of entity occurrences associated with one occurrence of related entity Established by very concise statements known as business rules

Relationship Strength Weak (non-identifying) relationships Exists if PK of related entity does not contain PK component of parent entity Strong (identifying) relationships Exists when PK of related entity contains PK component of parent entity

Weak Entities Weak entity meets two conditions Existence-dependent Primary key partially or totally derived from parent entity in relationship Database designer determines whether an entity is weak based on business rules

Relationship Participation Optional participation One entity occurrence does not require corresponding entity occurrence in particular relationship Mandatory participation One entity occurrence requires corresponding entity occurrence in particular relationship

Relationship Degree Indicates number of entities or participants associated with a relationship Unary relationship Association is maintained within single entity Binary relationship Two entities are associated Ternary relationship Three entities are associated

Associative (Composite) Entities Also known as bridge entities Used to implement M:N relationships Composed of primary keys of each of the entities to be connected May also contain additional attributes that play no role in connective process

University Teaching Database Design an E-R schema for a database to store info about professors, courses and course sections indicating the following: The name and employee ID number of each professor The salary and email address(es) for each professor How long each professor has been at the university The course sections each professor teaches The name, number and topic for each course offered The section and room number for each course section Each course section must have only one professor Each course can have multiple sections