FIT2094 Lecture Notes - Lecture 4: Data Modeling, Unique Key, Social Security Number
30 May 2018
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FIT2094 - Notes
Relationships: relationships describe associations among data, between
two entities
Composite and Simple Attributes:
Attributes are classified as simple or composite. A composite attribute, not
to be confused with a composite key, is an attribute that can be further
subdivided to yield additional attributes. For example, the attribute
ADDRESS can be subdivided into street, city, state and zip code.
Similarly, the attribute PHONE_NUMBER can be subdivided into area
code and exchange number. A simple attribute is an attribute that cannot
be subdivided. For example, age, sex, and marital status would be
classified as simple attributes. To facilitate detailed queries, it is wise to
change composite attributes into a series of simple attributes.
Single-Valued Attributes:
A single-valued attribute is an attribute that can have only a single value.
For example, a person can have only one Social Security number, and a
manufactured part can have only one serial number. Keep in mind that a
single-valued attribute is not necessarily a simple attribute.
Derived Attribute:
A derived attribute is an attribute whose value is calculated from other
attributes. The derived attribute need not be physically stored within the
database; instead, it can be derived by using an algorithm.
Multivalued Attributes:
Multivalued attributes are attributes that can have many values.
Existence Dependence:
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. In
implementation terms, an entity is existence-dependent if ti has a
mandatory foreign key -that is, a foreign key attribute that cannot be null.
If an entity can exist apart from one or more related entities, it is said to be
existence-indepent.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Relationships: relationships describe associations among data, between two entities. A composite attribute, not to be confused with a composite key, is an attribute that can be further subdivided to yield additional attributes. Address can be subdivided into street, city, state and zip code. Similarly, the attribute phone_number can be subdivided into area code and exchange number. A simple attribute is an attribute that cannot be subdivided. For example, age, sex, and marital status would be classified as simple attributes. To facilitate detailed queries, it is wise to change composite attributes into a series of simple attributes. A single-valued attribute is an attribute that can have only a single value. For example, a person can have only one social security number, and a manufactured part can have only one serial number. Keep in mind that a single-valued attribute is not necessarily a simple attribute. A derived attribute is an attribute whose value is calculated from other attributes.
