9
IF one, M (Mandatory) THEN participation in the relationship by the entity is mandatory, and at least one entity instance must participate in the relationship. True or False
True
IF zero, O (optional) THEN participation in the relationship by the entity is optional, and no entity instance must participate in the relationship. True or False
True
In a one-to-many relationship: -The entity on the one side of the relationship is called the parent entity or just the parent. -The entity on the many side of the relationship is called the child entity or just the child. True or False
True
Minimums are generally represented as either zero or one in E-R diagrams (but written with a O for optional or M for mandatory): True or False
True
Relationships connecting supertypes and subtypes are called IS-A relationships, because a subtype is a supertype. True or False
True
The ID-dependent entity is a logical extension or subunit of the parent: True or False
True
The principal difference between an entity and a table (relation) is that you can express a relationship between entities without using foreign keys True or False
True
The supertype contains all common attributes, while the subtypes contain specific attributes True or False
True
Two entities have a binary relationship of degree two. True or False
True
•If subtypes are exclusive, one supertype relates to at most one subtype. True or False
True
•If subtypes are inclusive, one supertype can relate to one or more subtypes. True or False
True
•Subtypes are used to avoid value-inappropriate nulls. (Why give a undergraduate student a null value for GMAT score? It's not that the test is missing from their records, it's that the test is inappropriate for admission as an undergrad. So to avoid this we create subtypes.) True or False
True
•The identifier of the supertype and all of its subtypes must be identical; i.e., the identifier of the supertype becomes the identifier of the related subtype(s). True or False
True
Identifiers
are attributes that name, or identify, entity instances.
Composite identifiers
are identifiers that consist of two or more attributes.
Relationship classes:
associations among entity classes
Relationship instances:
associations among entity instances
Information Engineering (IE)
by James Martin (1990); uses "crow's foot" notation, is easier to understand, and we will use it
Original E-R model
by Peter Chen (1976)
Unified Modeling Language (UML)
by the Object Management Group; it supports object-oriented methodology [see Appendix C]
Entity instance
the occurrence of a particular entity
The degree of the relationship is not the number of entity classes in the relationship True or False
False
Three entities have a ternary relationship of degree two True or False
False degree of three
The subtypes may have a discriminator attribute which indicates the subtype True or False
False supertype may have a discriminator attribute
Minimum cardinality
is the minimum number of entity instances that must participate in a relationship.
The Three Phases of Database Design are
Conceptual schema (E-R Model) Logical schema (relational model) Physical schema
All ID-Dependent entities are not considered weak True or False
False
There are three types of maximum cardinality:
One-to-One [1:1] One-to-Many [1:N] Many-to-Many [N:M]
Physical schema
Tables Columns and data types Database rules: e.g. referential integrity constraint Foreign Key / Primary key
A data model is a plan or blueprint for a database design. True or False
True
A data model is more generalized and abstract than a database design. True or False
True
All entity instances of a given entity class have the same attributes but change in the values of those attributes. True or False
True
Entity class
a collection of entities of a given type
IDEF1X
a national standard developed by the National Institute of Standards and Technology [see Appendix B]
Attributes
describe an entity's characteristics.
Extended E-R model
extensions to the Chen model
Entity-Relationship model
is a set of concepts and graphical symbols that can be used to create conceptual schemas.
A subtype entity
is a special case of a supertype entity:
ID-dependent entity
is an entity (child) whose identifier includes the identifier of another entity (parent).
A weak entity
is an entity whose existence depends upon another entity.
Maximum cardinality
is the maximum number of entity instances that can participate in a relationship.
What is the advantage of learn E-R model
•Communication - between developers •Documentation—for maintenance/future projects and for liability...we may need to show standard design practices were used in the construction of the databases
Conceptual schema (E-R Model)
•Entities •Attributes •Relationships and Cardinality •Identifiers
Logical schema (relational model)
•Relations •Attributes •Functional dependencies •Determinants
Cardinality
•Relationships in the E-R model are classified by their cardinality •means "count" and is expressed as a number
