Student notes 5.2. The DBMS. Security. Integrity

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1 Student notes 5.2 The DBMS Modern databases are controlled by a database management system (DBMS). This is software that acts as an intermediary between the data and the applications that the organisation needs. The DBMS can exert all sorts of control over the handling of the data that helps to maintain its security and integrity. Security This is to do with keeping data safe. It must be kept safe from: malicious access it might be copied or deleted by a disgruntled employee or a hacker; loss due to accidents it might be deleted by software or hardware failure or by fire or flood. Integrity Data integrity is the state of keeping data in such a way that it is correct, reliable and as it is intended. Integrity can be compromised if unintended changes take place by accident or by malicious individuals. An organisation usually has many data handling applications that deliver the information required to the users. For example, the human resources department needs to see information about staff, such as their salaries, positions, names and addresses and other contact details, plus work records. The accounts department needs to see details about sales, orders, payroll and tax transactions. Underneath all of this is the company s database that stores all the data that the company needs. The DBMS is there to protect the data and make it possible to fulfil all the requirements that are made on it. The applications have to work through the DBMS and are never allowed to handle the data directly. Application 1 Application 2 Application 3 DBMS Operating system Database Disk Page 1 of 8

Database levels Databases are complex things and they can be understood at different levels.

It is the front end of the application that gives appropriate access to the underlying data.

The conceptual view This is concerned with how the data is organised. The designer of the database has control over this.

2 Database levels Databases are complex things and they can be understood at different levels. These reflect the reality of a database as seen from different points of view. The external view This is what the user sees. It is the front end of the application that gives appropriate access to the underlying data. It will probably give a restricted view of the data so that the user only sees what is necessary to do the job. The conceptual view This is concerned with how the data is organised. The designer of the database has control over this. It is concerned with table design and the links between tables. The physical view The data resides on a storage medium of some sort. Accessing this is the job of the DBMS, acting through the operating system. Designers and users do not need to know anything about this. Page 2 of 8

Types of database Flat file Flat file databases are the oldest and simplest type of database. They are made up of just one table. Each row is a record and each column is a field.

Hierarchical database Sometimes the nature of the job to be done with a database is suited to a tree-like structure.

3 Types of database Flat file Flat file databases are the oldest and simplest type of database. They are made up of just one table. Each row is a record and each column is a field. For simple applications, this might be sufficient, but it soon causes problems if the use of the database expands. Hierarchical database Sometimes the nature of the job to be done with a database is suited to a tree-like structure. This is suitable if items are arranged as sub-parts of larger items. It works well for components in a car, for example, where the car contains an engine, which contains pistons and cam shafts etc. It is a good design of database where it is necessary to find appropriate components of larger objects. Page 3 of 8

4 Database software Applications are needed to let users work with databases in an efficient way. DBMSs provide ways of creating applications. The advantage of using a DBMS to create an application is that it will not allow inconsistent actions to be taken and protects the data to some extent from bad software design. Most DBMSs provide a database creation and query language, usually SQL (Structured Query Language), which allows the safe creation of applications. Here are some simple examples of SQL code. The DBMS must allow certain basic facilities to users, so that they can do their work and that the data is not damaged. The DBMS must allow data sharing. Most databases have multiple users and have multiple applications working on them. It is necessary to hold just one copy of data that everyone can access this prevents inconsistent updates. The DBMS must make it possible to change the data in a database without having to change the applications. The DBMS must be able to provide multiple views of the same data. Different people need different subsets to work on. They must all, however, be able to see the one and only up-to-date copy of the data, so that there are no inconsistencies. ACID There is a set of rules that databases should always be subjected to so that inconsistent changes are not made. These are known as the ACID rules. They refer to transactions: actions that bring about a change to the data. Atomic a change is either completely performed or not at all. It must not be possible to abandon a transaction and save it in a half completed state. For example, in a bank, if one account is debited, another must be credited or the money just disappears. Consistent a transaction must take a database from one consistent state to another. Again, in a banking situation, the total amount of debits in the complete system must equal the total credits. Isolated a transaction should not be visible to any other transaction or user until it Page 4 of 8

5 is irrevocably committed. Imagine if several theatre booking offices had access to a particular seat in a particular theatre on a particular day, all at the same time: double bookings could easily occur. It is necessary to lock a record while a transaction is taking place and not release it until it has been committed. Durable once a change has been made, it must not be lost due to any subsequent failure. Referential integrity Some changes to a database might not make sense. For example, in a school, there might be information about which pupils are taking which exams. If someone deleted an exam subject and there were pupils entered for it, this would make it impossible to do the entry processing. DBMSs allow the database designer to enforce referential integrity which prevents inconsistent changes from being carried out. Applications DBMSs allow the creation of applications. The DBMS makes sure that any applications created are not able to damage the underlying data. Applications can be created by the language provided or often by using a visual interface. This allows the designer to create objects such as: tables forms queries reports modules by manipulating objects on a screen. It is often possible to create the database objects using a wizard. This asks questions about what is required and then constructs the component automatically. Tables These are the essential data stores of the database. The fields can be created manually or a wizard can suggest fields for you. It is important to make the right decisions about which data goes into which table. Also, data types have to be chosen so that they can help in the validation of data input and ensure the database performs in the way intended. Page 5 of 8

Queries These are used to extract a subset of the data.

6 Forms These are the interface between the user and the underlying application. They can accept data as well as display it. They can have controls added, such as buttons and combo boxes, in order to provide interaction and validation capability. Queries These are used to extract a subset of the data. They can be applied to more than one table in order to provide the user with exactly the right data to work on. Queries can be constructed visually as in the screenshot on the next page. The tables and fields are inserted where required. The conditions can be added by filling in boxes. Page 6 of 8

Alternatively, SQL can be used to construct a query. This is useful if you need to use it in a program. The above query looks like this in SQL view: SELECT tblbooking. date AS date, tblbooking.

7 Alternatively, SQL can be used to construct a query. This is useful if you need to use it in a program. The above query looks like this in SQL view: SELECT tblbooking. date AS date, tblbooking. room_number AS room_number, tblbooking. client_ref AS client_ref, tblbooking. charge AS charge, tblbooking. num_ nights AS num_nights, tblclient. last_name AS last_name, tblroom. view AS view FROM tblroom, tblbooking, tblclient WHERE tblroom. room_number = tblbooking. room_number AND tblclient. client_ref = tblbooking. client_ref AND tblroom. view = True ORDER BY date ASC Reports These are printed lists of selected data from a database. They can be set to present data in all sorts of ways. They can be based on tables or queries in order to select exactly the data that is needed. This is the design screen for a report: Page 7 of 8

8 Here is the output from that report: Modules These are units of program code. Most DBMSs have a programming language built in so that a developer can have total control over the performance of the applications. For example, Microsoft Access has the language VBA (Visual Basic for Applications) built into it. Here is a short example of VBA code. Sub RunMyList() Dim MyDB As Database, MyRecs As Recordset, MyName As String Set MyDB = CurrentDb() Set MyRecs = MyDB.OpenRecordset( MyList ) MyName = InputBox( Enter your name ) MyRecs.MoveFirst Do While Not MyRecs.EOF Page 8 of 8

CSE 3241: Database Systems I Databases Introduction (Ch. 1-2) Jeremy Morris

CSE 3241: Database Systems I Databases Introduction (Ch. 1-2) Jeremy Morris 1 Outline What is a database? The database approach Advantages Disadvantages Database users Database concepts and System architecture