انت هنا الان : شبكة جامعة بابل > موقع الكلية > نظام التعليم الالكتروني > مشاهدة المحاضرة

Normalization

الكلية كلية تكنولوجيا المعلومات     القسم قسم البرامجيات     المرحلة 2
أستاذ المادة مازن كاظم حميد المنصوري       18/01/2016 09:36:20
NORMALIZATION
Is a process for evaluating and correcting table structures to minimize data redundancies, thereby reducing the likelihood of data anomalies.

Normalization works through a series of stages called normal forms. The first three stages are described as first normal form (1NF), second normal form (2NF) and third normal form (3NF).
From a structural point of view, 2NF is better than 1NF and 3NF is better than 2NF.

Denormalization
Produces a lower normal form, that is a 3NF will be converted to a 2NF through
denormalization
A successful design must also consider end-user demand for fast performance. Therefore you will occasionally be expected to denormalize some portions of database design in order to meet performance requirements
THE NEED FOR NORMALIZATION
In following example


We see in that example the structure of data set does not conform to the requirements of table nor does it handle data very well.
Consider the following deficiencies:

1. The project number (PROJ_NUM) is apparently intended to be primary key or at least a part of a PK, but it contains nulls.
2. The table entries invite data inconsistencies. For example the JOB_CLASS value "Elect. Engineer" might be entered as "Elect. Eng."
3. The table displays data redundancies. Those data redundancies yield the following anomalies:

a. Update anomalies. Modifying the JOB_CLASS for employee number 105 requires (potentially)many alterations, one for each EMP_NUM=105.
b. Insertion anomalies. Just to complete a row definition, a new employee must be
assigned to a project. If the employee is not assigned, a phantom project must be
created to complete the employee data entry
c. Deletion anomalies. Suppose that only one employee is associated with a given project, if that employee leaves the company and the employee data are deleted , the project information will also be deleted .to prevent the loss of the project information ,a fictitious employee must be created just to save the project information.


المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .