Fichas
Databases Ch3
Fichas del conjunto (19)
- 1These are specific things or objects in the mini-world that are "important enough to keep information about"Entities
- 2These are properties used to describe entitiesAttributes
- 3ER (Entity Relationship) model describes data as:-Entities
-Relationships
-Attributes - 4What are the different categories of an attribute?-Simple
-Composite
-Multi-valued
-Complex
- Derived - 5Describe a simple attributeWhen there is a single, atomic value for an attribute (SSN, Date of birth)
- 6Describe composite attributesThe attribute may be composed of several components (address (street, city, zip), Name (First, middle, last))
- 7Describe a multi-valued attibutean entity may have multiple values for that attribute. (color of a CAR of PreviousDegrees of a STUDENT)
Denoted as {Color} or {PreviousDegrees} - 8Describe a Complex attributein general, composite and multi-valued attributes may be nested arbitrarily to any number of levels, although this is rare
PreviousDegree of a STUDENT is a composite multi-valued attribute denoted by {PreviousDegrees(College, Year, Degree, Field)} - 9Describe a Derived AttributeWhen the value of an attribute can be determined from the values of one or more other attributes
ex: age is derived from Date of Birth - 10What are the 3 reasons why an entity may not have a value for an attribute?-Attribute does not apply in all cases (Apt number)
-Don't know if the value exists (home phone number)
-We know the value should be present but is missing (height of a person) - 11What is the degree of the relationship type?it is the number of participating entities in the relationship
-Binary
-Ternary
-N-ary - 12What are the 2 types of structural constraints on relationship types?Carnality Ratio
Participation Constraint - 13What is the cardinality Ratio?specifies the maximum number of relationship instances an entity can participate in.
- one-to-one
- one-to-many/many-to-one
-many-to-many - 14What is participation constraint?Specifies minimum participation (minimum cardinality constraint)
-Total participation - if the existence of the entity depends on it being related to another entity
-partial participation - if some entities are related to another entity but not all - 15Describe a recursive relationship typeA relationship type between the same participating entity type in distinct roles.
-Supervisor
-Supervisee - 16In a recursive relationship, what must be present in the lattice/relationship diagram?the numbers 1 or 2 to show if they are the supervisor or the subordinate
- 17What is the alternative notation of relationship structural constraints?(min, max)
-written on the edge in parenthesis
-Read the min, max numbers next to the entity - 18Describe what a weak entity type is-An entity that does not have a key attribute and that is identification-dependent on another entity type
-Identify this with a partial key doted underline
-a weak entity must participate in an identifying relationship type with an owner or identifying entity type
-must have total participation - 19Some tool kits cannot directly represent n-ary relationships. How would you break it down to appease the tool kit?In some cases, a ternary relationship can be represented as a weak entity if the data model allows a weak entity type to have multiple identifying relationships