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Subject: COM130 as*ingnment one(1)
Replies: 0 Views: 2125
14u 25.02.08 - 11:54am
Problems addressed by normalization
An update anomaly. Employee 519 is shown as having different addresses on different records.
An insertion anomaly. Until the new faculty member is assigned to teach at least one course, his details cannot be recorded.
A deletion anomaly. All information about Dr. Giddens is lost when he temporarily ceases to be assigned to any courses.A table that is not sufficiently normalized can suffer from logical inconsistencies of various types, and from anomalies involving data operations. In such a table:
The same information can be expressed on multiple records; therefore updates to the table may result in logical inconsistencies. For example, each record in an Employees' Skills table might contain an Employee ID, Employee Address, and Skill; thus a change of address for a particular employee will potentially need to be applied to multiple records (one for each of his skills). If the update is not carried through successfullyif, that is, the employee's address is updated on some records but not othersthen the table is left in an inconsistent state. Specifically, the table provides conflicting answers to the question of what this particular employee's address is. This phenomenon is known as an update anomaly.
There are cir tances in which certain facts cannot be recorded at all. For example, each record in a Faculty and Their Courses table might contain a Faculty ID, Faculty Name, Faculty Hire Date, and Course Codethus we can record the details of any faculty member who teaches at least one course, but we cannot record the details of a newly-hired faculty member who has not yet been assigned to teach any courses. This phenomenon (fact/ event) is known as an insertion anomaly.
There are cir tances in which the deletion of data representing certain facts necessitates the deletion of data representing completely different facts. The Faculty and Their Courses table described in the previous example suffers from this type of anomaly, for if a faculty member temporarily ceases to be assigned to any courses, we must delete the last of the records on which that faculty member appears. This phenomenon is known as a deletion anomaly.
Ideally, a relational database table should be designed in such a way as to exclude the possibility of update, insertion, and deletion anomalies. The normal forms of relational database theory provide guidelines for deciding whether a particular design will be vulnerable to such anomalies. It is possible to correct an unnormalized design so as to make it adhere to the demands of the normal forms: this is called normalization.
Normalization typically involves decomposing an unnormalized table into two or more tables that, were they to be combined (joined), would convey
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