Database Chp. 4

required vs. optional attributes

Required must have a value, cannot be left empty. Optional does not require a value and call be left empty.

connectivity

describes the relationship classification. "speak" the relationship in both directions to determine connectivity direction (1:M usually) (line endings on CF ERD)

optional relationship participation

one entity occurrence does not require a corresponding entity occurrence in a particular relationship (0:1 or 0:N cardinality)

mandatory relationship participation

one entity occurrence requires a corresponding entity occurrence in a particular relationship (1:1 or 1:N cardinality)

attribute identifier

one or more attributes that uniquely identify each entity instance

What are the entities that participate in a relationship called?

participants

composite identifier

primary key composed of more than one attribute

strong (identifying) relationship

primary key of the related entity contains a primary key component of the parent entity (solid line). Existent dependent entities automatically have this

weak (non-identifying) relationship

primary key of the related entity does not contain a primary key component of the parent entity (dashed line)

recursive relationship

relationship exists between occurrences of the same entity set

attribute domain

set of possible values for a given attribute (Sex: M or F, GPA: 0 -> 4.0)

derived attribute

value is calculated from other attributes and may not physically be stored (ie: class rank from num hours)

What do you have to do to multivalued attributes to make them function correctly in a database?

you may need to separate it into multiple different attributes or make a new entity composed of the original attributes components (most common)

binary relationship

2 entities are associated

ternary relationship

3 entities are associated

What are the main components of entity relationship components?

Cardinality, connectivity, entities, relationships, identifying/non-identifying

composite attribute

Attribute that can be subdivided to yield additional attributes (ie, fullname)

What does an ERD depict?

Conceptual database as viewed by the end user and the databases main components (entities, attributes, and relationship)

In a relational environment, does an entity correspond to a table or a row?

Corresponds to a table, not a row in the relational environment.

existence dependence

Entity exists in the database only when it is associated with another related entity occurrence (ie, dependent entity with employee entity)

How are entity names written in Chen and Crow's foot ERDs?

Entity names are nouns written in capital letters, and are represented by a rectangle with the entity's name

weak entity

NOT the same as a weak relationship. must be existence dependent and have a primary key that is partially or totally derived from the parent entity. entity weakness is based on the business rules (decided by the designer)

Does the term "entity" refer to the entity set or a single entity occurrence?

Refers to the entity set and not to a single entity occurrence.

What are some advantages and disadvantages or storing derived attributes? Not storing?

Stored Advantages: saved processing cycles, data access time, tracking of historical data. Nonstored Advantages: saves storage space, current value always yielded Stored Disadvantages: constant maintenance to ensure derived value is current, especially if any values in the calculation are changed. Nonstored Disadvantages: uses processing cycles, increase data access time, adds coding complexity to queries.

Will real-life database design require the reconciliation of conflicting goals? Why?

Yes, because sometimes plans will interfere with other design elements, and one will have to be compromised for the sake of the other one.

unary relationship

associated is maintained within a single entity

relationships

association between entities that always operate in both directions

simple attribute

attribute that cannot be subdivided (ie: middle initial)

single-valued attribute

attribute that has only a single value (ie: SSN, birthdate)

multivalued attributes

attributes that may have many values (ie: dependents, job history)

Entity Relationship Model

basis of an entity relationship diagram

What are cardinality and connectivity determined by?

business rules

What is an attribute?

characteristic of an entity (columns within the tables)

associative (composite) entities

composite or bridge entities. used to break apart and represent a M:N relationship between 2+ entities. had 1:M relationships with the parent entities and is composed of the primary key attributes of each parent (may also include additional attributes)

How are relationships between entities defined, refined, and incorporated into the database design process?

create a detailed narrative, identify the business rules, identify the main entities and relationships from those rules, develop an initial ERD, identify attributes and primary keys, then review and revise the ERD

existence independence

entity exists apart from all of its related entities. referred to as a strong or regular entity (ie, list of majors available at a school)

cardinality

expresses the minimum and maximum number of entity occurrences associated with one occurrence of a related entity (numbers below connections on CF ERD)

relationship degree

indicates the number of entities or participants associated with a relationship