7,540
edits
Changes
→Fontos fogalmak
=Fontos fogalmak=
==Edgar F. Codd==
https://en.wikipedia.org/wiki/Edgar_F._Codd<br>
Edgar Frank "Ted" Codd (19 August 1923 – 18 April 2003) was an English computer scientist who, while working for IBM, invented the relational model for database management, the theoretical basis for relational databases and relational database management systems. He made other valuable contributions to computer science, but the relational model, a very influential general theory of data management, remains his most mentioned, analyzed and celebrated achievement
==Segéd fogalmak==
https://en.wikipedia.org/wiki/Database_normalization#Normal_forms<br>
Database normalization is the process of restructuring a relational database in accordance with a series of so-called normal forms in order to reduce data redundancy and improve data integrity. It was first proposed by Edgar F. Codd as an integral part of his relational model.
When an attempt is made to modify (update, insert into, or delete from) a relation, the following undesirable side-effects may arise in relations that have not been sufficiently normalized:
* '''Update anomaly'''. The same information can be expressed on multiple rows; therefore updates to the relation may result in logical inconsistencies.
* '''Insertion anomaly'''. There are circumstances in which certain facts cannot be recorded at all. For example, each record in a "Faculty and Their Courses" relation might contain a Faculty ID, Faculty Name, Faculty Hire Date, and Course Code. Therefore, we can record the details of any faculty member who teaches at least one course, but we cannot record a newly hired faculty member who has not yet been assigned to teach any courses, except by setting the Course Code to null. This phenomenon is known as an insertion anomaly. :[[File:ClipCapIt-181007-123437.PNG]]
* '''Deletion anomaly'''. Under certain circumstances, deletion of data representing certain facts necessitates deletion of data representing completely different facts
==Denormalization==
=Bevezető=