OCR GCSE Computing © Hodder Education 2013 Slide 1 Chapter 5: DBMS OCR GCSE Computing © Hodder Education 2013 Slide 1

OCR GCSE Computing © Hodder Education 2013 Slide 2 Chapter 5: DBMS Most databases are manipulated with a Database Management System (DBMS). A DBMS provides the software tools to create, maintain and interrogate a database. The physical data is handled by the DBMS. The database programmer writes applications software that works through the DBMS. OCR GCSE Computing © Hodder Education 2013 Slide 2

OCR GCSE Computing © Hodder Education 2013 Slide 3 Chapter 5: DBMS The DBMS must: allow data sharing; allow multiple user access; allow multiple applications to work on them; hold just one copy of data that everyone can access – this prevents inconsistent updates; make it possible to change the data in a database without having to change the applications. OCR GCSE Computing © Hodder Education 2013 Slide 3

OCR GCSE Computing © Hodder Education 2013 Slide 4 Chapter 5: DBMS The DBMS must: provide multiple views; present data to different users in different ways: The human resources department needs full personal details of employees. A co-worker might just need to see a colleague’s telephone number. OCR GCSE Computing © Hodder Education 2013 Slide 4

OCR GCSE Computing © Hodder Education 2013 Slide 5 Chapter 5: DBMS The DBMS must protect data from inconsistent requirements. Many databases are used by more than one user at a time. What if two users try to update the same data at the same time? Suppose a theatre ticket is being sold by one agency and another agency tries to sell the same ticket at the same time. You can get double booking. The transaction must be locked while the record is updated, so that other users cannot access it. OCR GCSE Computing © Hodder Education 2013 Slide 5

OCR GCSE Computing © Hodder Education 2013 Slide 6 Chapter 5: DBMS All database transactions must be subject to the ACID test. They must be: Atomic – a change is either completely performed or not at all; Consistent – a transaction must take a database from one consistent state to another Isolated – a transaction should not be visible to any other transaction or user until it is irrevocably committed Durable – once a change has been made, it must not be lost as a consequence of any subsequent failure. OCR GCSE Computing © Hodder Education 2013 Slide 6

OCR GCSE Computing © Hodder Education 2013 Slide 7 Chapter 5: DBMS Referential integrity Different applications must not cause conflicts. Records must be locked while one application is performing an update so that another application cannot produce an inconsistent result. Inconsistent changes must be prevented – for example , an exam subject must not be deleted if there are students connected to it. This is called maintaining referential integrity. OCR GCSE Computing © Hodder Education 2013 Slide 7

OCR GCSE Computing © Hodder Education 2013 Slide 8 Chapter 5: DBMS Applications The DBMS must provide ways of creating applications. Most DBMS software has a language for creating applications. Most support some form of SQL (Structured Query Language). SQL has features for creating, changing and interrogating a database. Many also have a visual interface for creating queries and other objects. OCR GCSE Computing © Hodder Education 2013 Slide 8