Ch 4 Flashcards
Required attribute
A required attribute is an attribute that must have a value, i.e. it cannot be left empty.
Optional attribute
An optional attribute is an attribute that does not need to have a value, i.e. it can be left empty.
Identifiers
One or more attributes that uniquely identify each entity instance.
Composite identifier
A primary key composed of more than one attribute.
Composite attribute
A composite attribute, not to be confused with a composite key, is an attribute that can be further subdivided to yield additional attributes.
Simple attribute
A simple attribute is an attribute that cannot be subdivided.
Single-valued attribute
A single-valued attribute is an attribute that can only have a single value. E.g. social security numbers, serial numbers.
Multivalued attributes
Multivalued attributes are attributes that can have many values. E.g. college degrees (you can have many different ones).
Derived attribute
A derived attribute is an attribute whose value is derived (calculated) from other attributes.
Participants
The entities that participate in a relationship are known as participants.
Cardinality
Cardinality expresses the minimum and maximum number of entity occurrences associated with one occurrence of the related entity.
Existence-dependency
An entity is said to be existence-dependent if it can exist in the database only when it is associated with another related entity occurrence.
Existence-independence
If an entity can exist apart from all of its related entities, then it is existence-independent.
Strong/regular entity
Entities that are existence-independent are strong (or regular) entities.
Weak/non-identifying relationship
A weak relationship, also known as a non-identifying relationship, exists if the primary key of the related entity does not contain a primary key component of the parent entity.
Strong/identifying relationship
A strong relationship, also known as an identifying relationship, exists when the primary key of the related entity contains a primary key component of the parent entity.
Weak entity
A weak entity is one that meets two conditions:
1) The entity is existence-dependent; it cannot exist without the entity with which it has a relationship.
2) The entity has a primary key that is partially or totally derived from the parent entity in the relationship.
Optional participation
Optional participation means that one entity occurrence does not require a corresponding entity occurrence in a particular relationship.
Mandatory participation
Mandatory participation means that one entity requires a corresponding entity occurrence in a particular relationship.
Relationship degree
A relationship degree indicates the number of entities or participants associated with a relationship.
Unary relationship
A unary relationship exists when an association is maintained within a single entity.
Binary relationship
A binary relationship exists when two entities are associated.
Ternary relationship
A ternary relationship exists when three entities are associated.
Recursive relationship
A recursive relationship is one in which a relationship can exist between occurrences of the same entity set (naturally, such a condition is found within a unary relationship).
Steps to build an ERD
- Create detailed narrative of organization’s descriptions of operations.
- Identify the business rules based on the description of operations.
- Identify the main entities and relationships from the business rules.
- Develop the initial ERD.
- Identify attributes and primary keys that adequately describe the entities.
- Revise and review the ERD.
