相关数据库于海清outcome1.doc

(1) When designing a database system, an entity is an object of importance about which you wish to store information. (2) For example, in a student registration system, entities of importance could be---student, course, unit, and so on. The entities are company, customer, car, van, rate, discount scheme (1) An attribute is an item of data, which is stored about an entity occurrence. (2) For example, in a student registration system, the entity student could bave attributes such as student ID, surname, forename, date of birth, telephone number, etc. The four entities are: Employee (Employee No, Employee Name, gender, address, date of birth) Department (Department No, Department Name) Qualification (Qualification NO, Qualification Name) Skill (Skill No, Skill type) (1) An entity occurrence is an instance of an entity. (2) For example, in a student registration system, the information about each individual student is an entity occurrence. (1) A primary key comprises one or more attributes in an entity that uniquely identifies an entity occurrence. (2) For example, an employee no uniquely identifies a member of staff within a company (1) An entity may have one or more choices for the primary key. Collective these are known as candidate keys. One is selected as the primary key. Those not selected are known as alternative key. (2) For example, an employee has an employee has an employee number, a National Insurance (NI) number and an email address. If the employee number is chosen as the primary key then the NI number and email address are secondary keys. (1) The key is foreign key (2) clientNo cName CR76 John Kay CR56 Aline Stewar clientNo propertyNo PAddress rentStart rentFinish Rent ownerNo oName CR76 PG4 6Lawrence St,Glasgow 1-Jul-00 31-Aug-01 350 CO40 Tina Murphy CR7 6 PG16 5Novar Dr, Glasgow 1-Step-01 1-Step-02 450 CO93 Tony Shaw CR56 PG4 6Lawrence St,Glasgow 1-Step-99 10-June-00 350 CO40 Tina Murphy Diagram -2 From the d