2. 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)

5. 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

6. 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

7. 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

8. 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

9. 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 ?

10. 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

12. 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

13. 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

14. 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

15. 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

17. 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

19. 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

21. 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

22. 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

24. 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

27. 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

30. 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

33. 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

35. 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

38. 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 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