Chapter 3 Flashcards
Attribute meaning ?
Used to describe entity
Types of attributes:
Simple(can only be one way or the other), composite( may have several components e.g address), multi value(entity may have multiple values for that attribute)
What is a key attribute?
An attribute which is unique to that one.
Composite key?
Can be unique but more parts to it e.g a number plate)
Multiple key?
More than one key. Entity that has more than one thing that makes it unique
Types of constraint on relationship.
Cardinality, specifies maximum participation : 1to1 1to m to one. M to n
Existence dependency: minimum participation. Zero and one or more
What is a recursive relationship?
Relationship between the same entity type in distinct role.
What is weak entity
Entity dosent have key attribute.
Data modelling tools advantage and disadvantage.
Advantage: easy user interface,
Negative mostly respreseny relation instead of ER.
What’s the difference between er and err?
Er dosent support specialization and generalization
What is a superclass?
The type of entity that everything goes back to. Employee would be superclass for secretary,technician.
Is a relationship is between?
A subclass and superclass
What is specialization?
Defining a set of subclasses of a superclass by a distinguishing characteristic of the entity in superclass.
Generalization
When several classes with common feature share a feature we generalize into a superclass: e.g car, truck become subclass to super class vehicle
What is Heirachy in err
When subclass can only have 1 superclass single inheritance.
What is lattice in err?
When subclass can Moore than one super class. Multiple inheritance
What are tuples?
Rows are called tuples
What is in colonm header of relational model?
Attribute
