Introduction Objectives of the Lecture : To consider “What is a Database ?” To consider “What is a Relational Database ?” i.e. what is a “relation”, and therefore what differentiates a relational database from any other kind of database.

What is a Database ? Definition : “A collection of data that is permanently stored on a computer”. In a database we should be able to : l have different types of data in the collection; l record relationships between different data items; l have varying sizes of data collections to suit our needs. We need to be able to : l insert new data, delete old data, and amend existing data in the collection; l retrieve data from the collection; l manage the collection so that it can be permanently stored in the face of various hazards that would otherwise corrupt or lose data in it.

Permanent Data Storage By “permanent” is meant that once data is put into the computer, it stays there until someone explicitly removes it, or until the computer is damaged or broken and this causes the data to be lost. The data is not lost when the computer is switched off or the database is not used for a long time. A computer has 2 classes of data storage : l Random Access Memory (= RAM). This is used by the computer’s Central Processing Unit (= CPU) to temporarily hold data that the CPU is processing. l Backing Store. This is used to store data permanently. Typically magnetic discs are used for this purpose, although other types of storage device, e.g. compact discs and magnetic tapes, are also used. Thus the database invariably uses the backing store to make the data storage permanent.

Different Types of Data l Numbers l Dates l Audio l Videos l Text l Pictures l Maps l etc. 1, -999, January 2004 ‘Janet & John’ To store useful information, many types may be needed together. Example : recording observations of a species of bird. Number of birds observed, textual description of characteristics, dates of observations, map of migration routes, pictures of birds, audio recording of bird calls, video of flight. There are many kinds or types of data, such as :

Relationships Between Data In practice, we are not interested in just isolated items of data. We also want to know the relationships between the data items. Example : species of bird observations It is important to know which bird numbers, descriptions, etc relate to which bird species. It may also be useful to know which bird species have similar migration routes. All sorts of relationships are possible : some essential to make sense of data, some useful for different purposes.

Quantities of Data Amounts of data may be large or small: l Example (small) : Names and addresses of members of a golf club (say 500 members). l Example (large) : Electricity company stores all customers’ account details (say name & address, a/c no., meter reading, amount owed, payment method, …. ; 1,000,000 customers). l Example (very large) : Supermarket stores customer purchase details for analysis (say average 10 purchases × 1000 customers × 400 stores × 355 days × 5 years = 7.1 billion purchase details).

Computerised Databases What is the benefit of a computerised database (= DB) compared to a manual paper-based collection of data ? l Compact. Less bulky than filing cabinets. l Speedy. Computer can retrieve and change data faster than a human being. l Less drudgery. Removes tedium. l Current. Easier to ensure data is up-to-date. Often databases are shared by several/many users. Then there are further benefits : l Consistency. All users have the same data. l Centralised management. The data collection can be properly managed on behalf of the whole organisation. Sharing consistent and well-managed data is much easier to achieve if the database is computerised.

Database Management Systems l Need special software to manage the DB if the benefits are to be obtained. l Don’t want to have to write our own computer programs to do this. Therefore buy a Database Management System (= a DBMS). A DBMS should provide the means to : l change the DB contents to keep them up-to-date; l find and retrieve data from the DB; l protect the data from accidents and intruders; l store it in an efficient way on the computer.

What is a Relational DB ? Definition : “A DB in which all the data is stored in relations”. A relation is a simple logical structure which contains related data (hence the name “relation”). It can be pictured as a table of data M-S S M D 8M EName Smith Mitchell Robson 8Blake ENo E3 E5 E1 4E Sal ,500 21,000 32, ,000 Example : A relation storing the employee numbers (= ENo), employee names (= EName), marital statuses (= M-S), and salaries (= Sal) of 4 employees. Row data is related because it all refers to the same employee. The rows are related because they all relate to employees.

A Relation The example relation has4 attributes, depicted as columns 4 tuples, depicted as rows M-S S M D 8M EName Smith Mitchell Robson 8Blake ENo E3 E5 E1 4E Sal ,500 21,000 32, ,000 As relations are just structures, they can hold any kind of data. So they can hold all the different kinds of data mentioned earlier, and more besides, without limit.

Relational Databases l Relations themselves are based on simple mathematical principles. l Relations can be manipulated by users in ways that are conceptually simple. l A relational DB is perceived and used as a collection of relations. l The relational DBMS manages the physical processing of data so that the user doesn‘t need to know what underlying computer processing goes on and merely perceives things in terms of relations.

Importance of Relational DBMSs l Because relational databases are built on sound theoretical mathematical principles, the theory is very practical. l It makes relational databases easy to learn and use. l Nevertheless relational databases can be very powerful and flexible. Therefore relational databases : l are the most common type of database on the market (over 90% share); l are extremely important in practice. Commercial relational DBMSs usually use a database programming language called SQL. (Thus sometimes they may be called SQL DBMSs).

Terminology l Because relations are usually depicted as tables, the word table is used in SQL instead of relation. l Consequently, as tables have columns and rows, the following tabular names are used in SQL (and often more generally) instead : tableinstead ofrelation columninstead ofattribute rowinstead oftuple Note 1 : some relations are better represented as graphs or networks. Note 2 : don’t confuse a relation with its representation.

Relational Implementations Principles Graphical Text QBE SQL Oracle MS Access MS SQL Server Calculus Algebra Domain Tuple RAQUEL IBM’s DB2 Sybase Ingres

Relational Standards l QBE and relational algebra have not been standardised. l SQL has been standardised. Later standards include additions to SQL compared to earlier standards. l The SQL standards do not adhere completely to relational principles. l Vendors’ SQL implementations often don’t adhere completely to SQL standards, and so are even more ad hoc. l This course uses Oracle SQL, because it is popular in the database market, but it does not adhere completely to the SQL standards.

Other Data Models Hierarchical : data items (= facts) are arranged in a hierarchical structure. Codasyl/Network : data items are arranged in a network structure. Object-oriented : data items (= facts) are considered as : objects containing data, with their own ways of handling their internal data, usually arranged in a network structure. There have been databases that used other data models :-