ch 4
What two conditions must be met before an entity can be classified as a weak entity? Give an example of a weak entity.
An entity that displays existence dependence and inherits the primary key of its parent entity. For example, a DEPENDENT requires the existence of an EMPLOYEE.
What is a composite entity, and when is it used?
A composite entity is also known as a "bridge" entity. This "bridge" is used to handle the many-to-many relationships that the traditional entity could not handle. This entity lies between the two entities that are of interest and this composite entity shares the primary keys from both the connecting tables.
What is a strong (or identifying) relationship, and how is it depicted in a Crow's Foot ERD?
A relationship that occurs when two entities are existence-dependent; from a database design perspective, this relationship exists whenever the primary key of the related entity contains the primary key of the parent entity.
What two courses of action are available to a designer who encounters a multivalued attribute?
An attribute that can have many values for a single entity occurrence. For example, an EMP_DEGREE attribute might store the string "BBA, MBA, PHD" to indicate three different degrees held.
What is a derived attribute? Give an example. What are the advantages or disadvantages of storing or not storing a derived attribute?
An attribute that does not physically exist within the entity and is derived via an algorithm. For example, the Age attribute might be derived by subtracting the birth date from the current date. What are some advantages and disadvantages or storing derived attributes? Not storing? Stored Advantages: saved processing cycles, data access time, tracking of historical data. Stored Disadvantages: constant maintenance to ensure derived value is current, especially if any values in the calculation are changed.
What three (often conflicting) database requirements must be addressed in database design?
Desing standardsprocessing speedInformation requirements
Given the business rule "an employee may have many degrees," discuss its effect on attributes, entities, and relationships. (Hint: Remember what a multivalued attribute is and how it might be implemented.)
The number of entities or participants associated with a relationship. A relationship degree can be unary, binary, ternary, or higher.
Discuss the difference between a composite key and a composite attribute. How would each be indicated in an ERD?
composite attribute - An attribute that can be further subdivided to yield additional attributes. For example, a phone number such as 615-898-2368 may be divided into an area code (615), an exchange number (898), and a four-digit code (2368). Compare to simple attribute. composite key - is a combination of two or more columns in a table that can be used to uniquely identify each row in the table when the columns are combined uniqueness is guaranteed, but when it taken individually it does not guarantee uniqueness.
