A-level Computing/AQA/Paper 1/Fundamentals of databases/Entity relationship modelling

Primary keys Normalisation Databases will probably store data about a variety of things. For example, if we look at the tables required for an online store: Staff Products Receipts Customers These tables could sit on their own, but when we start to describe each of them we notice that they are related to each other. For example: The Receipt table records which product a customer has bought, and which date it was purchased on. This describes the Receipt record, and looking at its structure we see that the primary keys from other tables are included in it. Receipt(CustomerID, ProductID, DateTime, Total, StaffID) In other words we could say: A receipt has one customer A customer can have many receipts A receipt has one product A product can be part of many receipts A receipt was created by one staff member A staff member can create many receipts We can then draw this diagram like so: center|350px To link tables together and allow for us to query a database we use relationships. There are three types of relationship that you need to know. Each is shown below using 'crows foot notation' which is one of many ways to describe these relationships: Diagram Name Description 250px|center|An Author can write several Books, and a Book can be written by several Authors Many to Many An Author can write several Books, and a Book can be written by several Authors 250px|center|A Biological Mother can have many children, and a child can have only one Biological Mother One to Many A Biological Mother can have many children, and a child can have only one Biological Mother 250px|center|A Country has only Capital City, and a Capital City is in only one Country One to One A Country has only one Capital City, and a Capital City is in only one Country 1 Exercise: Relationships Describe the following relationships: 230px An animal has one genome A genome describes only one animal 230px A student has many classes A class has many students 230px A league has many teams A team is only in one league 230px A player answers many questions A question can be answered many times Each answer is only applicable to one question A player can give many answers Each answer has only one player contributing Draw the following relationships: A vehicle has many wheels, each wheel has one vehicle vehicle ---< wheel The relationship between a father and a child Father >--< Child (unless we are talking about biological fathers: Father--<Child ) The relationship between a politician and a nose politician ---- nose The relationship between an owner and a cat Cat >---< Owner (even though an Owner may own more than one cat, a cat might have more than one owner) The relationship between a car and a driver car --- driver (a car can only be driven by one driver, a driver can only drive one car at one time) The relationship between a unicycle and a wheel wheel --- unicycle The relationship between a house and a postcode House >--- Postcode (a house is assign one postcode, but that same postcode might be assigned to many houses)