1. INFO1408 Database Design Concepts Week 15: Introduction to Database Management Systems

2. This lecture: –Why do so many organisations use databases? Next week: –Special features of databases Exam: –You need to know and understand these two lectures for the Exam in MAY. 2

3. Managing a database system is complex Fortunately this is handled by software known as a Database Management System ( DBMS). 3

4. Some important features of a DBMS Moves data to and from physical data files Manages multiple users - sharing data Manages transactions as ‘all or nothing’ units of work Supports a Query Language to retrieve data from the database Manages back ups Provides security. 4

5. The three tier architecture model for databases Makes it easier for people to understand the database structure The database is considered as three levels or tiers As database designers we are interested in the top two tiers. We know what the bottom tier does but we don’t need to know how it works. 5

6. External layer Logical layer Physical layer What the user sees (screens and reports) How the database organises the data for people to view How the computer’s operating system stores the data (on disk) 6

7. Database File Database File Database File Database File Database File Internal Schema User View 1 User View 2 User View …. External layer Logical layer Physical layer Physical Data independence Logical Data independence IMPORTANT DIAGRAM - LEARN IT! 7

8. Physical Layer The files that actually contain the data Data could be stored over multiple disc drives – maximise performance by running in parallel The DBMS works with the computer’s operating system (OS) to efficiently manage the file storage Users do not need to know how this physical layer works - neither do we at this point (Microsoft Access is unusual in that everything is stored in one file – this limits the number of users, so it is not used for large enterprises). 8

9. Logical Layer The logical layer transforms the physical data into a common structure This ‘abstracted’ format is known as a schema We can visualise this as all the table types. 9

10. External Layer This is where users interact with the database through forms, reports and queries The DBMS handles the communication between users and the computer system through SQL Different users can be shown different forms and reports, depending on what is needed for their department of the organisation, or their job Special ‘views’ can be created so that users only see what they need (more next week). They will only know about the parts of the database they use. 10

11. Advantages of database processing Getting more information from the same amount of data –The data is stored in one place (the database) and the DBMS displays it to users in helpful ways Sharing Data –Data can be shared by many users. If one user changes the address of a customer it is instantly available to other users. DBMS prevents possible problems through locking (see next week). 11

12. More advantages of database processing Users do not have to be physically close to the data (unlike a paper system) Controlling redundancy –Organisation only needs one copy of each data item –Cannot have different versions of one data item (eg different spellings of a customer’s name) Data is more reliable and less confusing - known as data integrity. 12

13. More advantages of database processing Data Independence –Easy to make changes to the database when required - programmes that access the database do not need to know its structure –Eg we want to add e-mail addresses to the Customer file: in old systems you had to change all the programmes that used the Customer file - lots of work data independence lets us add a new field to the Customer table without changing all the programmes that use it. 13

14. More advantages of database processing Improved security –Give users permission to: Access data Change data –Views can hide things from users. 14

15. Referential Integrity Referential integrity controls the links between records You can set referential integrity when related fields have the same data type (an AutoNumber field can be related to a Number field). 15

16. How referential integrity works You can't enter a value in the foreign key field of a table if it doesn't exist in the primary key of the related table –For example, you can't assign an order to a customer who is not in the Customer table You can't delete a record from a table if matching records exist in a related table –For example, you can't delete an employee if there are orders assigned to the employee in the Order table You can't change a primary key value for a record that has related records –For example, you can't change an employee's ID in the Employee table if there are orders assigned to that employee in the Orders table. 16

17. Cascade Update and Delete For relationships with referential integrity enforced, you can say whether you want Microsoft Access to automatically cascade update and cascade delete related records When you try to delete records or update linked primary key values, Microsoft Access changes the related tables to allow your updates to go ahead, preserving referential integrity Example of cascade update: –if you change a customer's ID in the Customer table, the CustomerID field in the Orders table is automatically updated for that customer's orders so that the relationship isn't broken Microsoft Access cascades updates without displaying any message. 17

18. Example of cascade delete: –If you delete a customer from the Customer table, all the customer's orders are automatically deleted from the Orders table –Records in the Order Details table that are related to the Orders records are also automatically deleted When you delete records with the Cascade Delete Related Records check box selected, Microsoft Access does not always warn you that the related records will be deleted too. 18

19. Disadvantages of Database Processing Larger file size Increased complexity Greater impact of failure More difficult recovery. 19

20. Summary Important features of a DBMS The three tier architecture Advantages of database processing Referential integrity Cascade Update and Cascade Delete Disadvantages of database processing. 20