Information Management - 4 Flashcards
1
Q
Entity Relationship Model
A
- ER model is the basis of ER diagrm
- represents conceptual database viweed as endsuer
- Composed of: entities, relationship, attribute
2
Q
Entities
A
- refers to entity and not a single entity
- Corresponds to table and not row in relational environment
- In Chen and Crow’s diagram, entity name is writtten inside the rectangle
- Entity is noun with capital lettters
3
Q
Attribute
A
characteristics of an entity
4
Q
Chen notation
A
attribute is written in an oval conncted to entity
5
Q
Crow’s foot
A
attribute is within the attribute box below any rectangle
6
Q
Required attribute
A
must have a value
7
Q
Optional Attribute
A
may be left null
8
Q
Composite Identifier
A
primary key composed of one or more attribute
9
Q
Identifiers
A
one or more attributes that uniquely identify each entity instance
10
Q
Composite attribute
A
can be subdivided
11
Q
Simple attribute
A
cannot be subdivided
12
Q
Single-value attribute
A
can only have a single value
13
Q
Multivalued attribute
A
can have multiple values
14
Q
Derived Attribute
A
can be calculated from other attribute
14
Q
Relationship
A
- 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