Karteikarten
Databases Ch5
Karten in diesem Stapel (16)
- 1What is a Relation?A relation is a mathematical concept based on the ideas of sets
- 2What does a relation look like? and what does it contain?looks like a table of values
-contains a set of rows - 3Describe the Domain:Set of atomic values
- 4Describe what atomic is:Each value is indivisible
- 5How do you specify a domain?assign data type for each domain
- 6Denoted by R(A1, A2, ..., An) Made up of a relation name R and a list of attributes, A1, A2, ... , AnRelational Schema
- 7Name of a role played by some domain D in the relational schema RAttribute Ai
- 8Number of attributes n of its relation schemaDegree (or arity) of a relation
- 9Total number of values in domainCarinality
- 10-Relation state at a given time -Reflects only the valid tuples that represent a particular state of the real worldCurrent relation state
- 11Indicate roles, or interpretations, for the domainAttribute names
- 12Characteristics of RelationsOrdering of tuples in a relation
-Ordering of values within a tuple and an alternative definition of a relation
-alternative definition of a relation
-Use the first definition of relation - 13Describe the values in a RelationFlat relational model
-composite and multivalued attributes not allowed
-First normal form assumpution
Multivalued attributes
-Must be represented by seperate relations
Composite attributes
-Represented only by simple component attributes in basic relational model - 14What does the NULL value mean in a Relation?-The value is unknown
-the value exists but is not available
-Attribute does not apply to this tuple (also known as value undefined) - 15Describe what an Inherent/Implicit Constraint is:based on the relational model: the relational model does not allow an attribute to have a list as a value, a relation cannot have duplicate values
- 16Describe the schema based constraint, Domain constraint:every value in a tuple must be from the domain of its attribute - or it could be NULL, if allowed for that attribute