Databases

Organise this: Jenny is 17 and studies chemistry and physics. Tom is her best friend, he’s also 17. Her sister Jackie is friends with Sally, who is 18. Jackie is also 18. The oldest is Jim, who is 17 and also has Tom as a best friend. Jenny is engaged to be married to Tim, who is 119 and studies biology and maths. Sally and Jackie study Maths, but Sally also studies Sport, though Jackie studies three subjects, as she also takes French.

Probably the best way: Name Age Best friend Studying Note Jenny 17 Tom Chem & Phys Engaged to Tim, sister of Jackie Jennie No subjects given Sally 18 Jackie Maths and Sport Tim 119 Biology, Maths Age a problem? Maths, Sport, French Sister of Jenny

Note If people are best friends, then they should be connected There is no way to “lock” best friends Some people’s subjects get repeated, so that wastes storage space and typing time There are several ways to write the subjects You could wrongly enter nothing at all if it’s just a simple table No validation has been performed, especially in the case of Tim aged 119 & Jenny/Jennie

A database is A persistent, organised store of data. People depend on databases, many businesses cannot function without them. So, the database must be Accurate (correct) Up-to-date (current) Readily available to whoever needs access Protected from people who shouldn’t have access

Some definitions: Entity – the complete real-world object for which you need to hold data Instance – one of the entities, also called a record Attribute – a characteristic of an entity Values – word or number information about the entity Relationship – a connection between entities Table – the way that most databases hold data DBMS – a database management system – “a computer-based system to manage a database or collection of databases”

Examples: Entities: customers, employees, films, pupils Instance: you, me, “Rain Man”, you Attribute: address, gender, length, age Value: 1 Darwin Lane, start date, 45mins, 15 Relationship: is_customer_of Table: pupils table, teacher table (etc.) DBMS: SIMS

Typical Types of Data: Strings (text) – characters, usually up to 256 or 512 bytes long Numbers – as integers Real numbers – with a decimal point and maybe an displayed in a form of scientific notation Prices – in whichever currency you want Yes/No data (Boolean) Dates Times These are all simple types, you may also see records – a compound data type, consisting of several fields of different simple types

Give examples of each type: Text/string: Integer: Real: Currency Boolean Date Time

What information might you need to store about ….. A CD in a DJ’s record collection? A candidate entering for an exam? The same CD in a record shop like HMV? A part in a car parts database? An item in a supermarket database? A customer buying travel insurance? A pupil in a school database? A patient in a hospital database?

We want to avoid: Errors Lost data Insufficient data Inconsistent data Unauthorised access

Errors can result in: Embarrassment – sending bills to dead people, for example Financial loss – banks need everything correct Life-or-death situations like air traffic control or airport data or navigational data for planes and ships.

Aims of managing a database: Minimising storage requirements Minimising repetition or redundancy Fast entry of valid data Avoidance of some verification Fast and easy searches Prints in neat layouts as reports

Data security Data integrity Not just keeping out intruders, but also minimising the damage from: Natural catastrophes like fire and flood Accidental actions Malicious actions Data integrity Making sure the data is accurate / correct / trustworthy Needs validation Software design that avoids inconsistent data

Validation and verification Validation – checking that the data is valid – that is, it means something for the field it is entered into. You could check for: the right type the right length Right punctuation/syntax Sensible values

How to protect data: Making backups Mirror servers Off site storage Restricted access to buildings Passwords on the computer Audit trails (who did what, when)

Creating a database in Access:

Exercise Start “Access” Create a data table for one or more of these examples: