MIT Database Management Systems Lesson 03: Entity Relationship Diagrams

Transcription

1 MIT Database Management Systems Lesson 03: Entity Relationship Diagrams By S. Sabraz Nawaz Senior Lecturer in MIT, FMC, SEUSL & A.J.M.Hasmy FMC, SEUSL

2 ER - Model The entity-relationship (ER) data model allows us to describe the data involved in a real-world enterprise in terms of objects and their relationships and is widely used to develop an initial database design MIT 22033, Database Management Systems By: S.Sabraz Nawaz 2

3 E-R Model (Entities) The two main constructs of the Entity- Relationship model are Entities & Relationships An entity is an object in the real world that is distinguishable from other objects o e.g. Lecturer, Student, Subject, etc. A collection of similar entities is called an entity set o e.g. Lecturers, Students, Subjects, etc. ENTITY MIT 22033, Database Management Systems By: S.Sabraz Nawaz 3

4 E-R Model (Entities...) Example, an employee in a company database. e1 e2 e3 e4 e5 e6 EMPLOYEES Employee Entity set MIT 22033, Database Management Systems By: S.Sabraz Nawaz 4

5 E-R Model (Attribute) An entity is described using a set of attributes o e.g. Name, NIC, etc. All entities in an entity set have the same attribute Attribute MIT 22033, Database Management Systems By: S.Sabraz Nawaz 5

6 E-R Model (Attributes ) salary dob Example: name, id, age & salary are attributes in EMPLOYEES entity e1 e2 e3 e4 e5 e6 name id Employee Entity set EMPLOYEES salary name dob MIT 22033, Database Management Systems By: S.Sabraz Nawaz 6 id

7 E-R Model (Attributes ) The attributes are said to be composite if they can be split. Eg:- the entity Employee contains attribute 'name' which is a composite one name EMPLOYEES LastName FirstName MIT 22033, Database Management Systems By: S.Sabraz Nawaz 7

8 E-R Model (Attributes ) A multi-valued attribute of an entity is one which has multiple values. o Eg:- attribute phone numbers in employee entity. EMPLOYEES surname first_name phone MIT 22033, Database Management Systems By: S.Sabraz Nawaz 8

9 E-R Model (Attributes ) Some attributes that can be computed from other attributes are called derived attributes. o Eg:- age is derived from date of birth & current date EMPLOYEES dob name age MIT 22033, Database Management Systems By: S.Sabraz Nawaz 9

10 E-R Model (Attributes another sample) Customers MIT 22033, Database Management Systems By: S.Sabraz Nawaz 10

11 E-R Model (Domain) Each attribute in an entity set has domain; possible values o e.g. Age attribute of an employee has possible integer values from MIT 22033, Database Management Systems By: S.Sabraz Nawaz 11

12 E-R Model (Keys) A Key is a minimal set of attributes whose values uniquely identify an entity in the set salary dob e1 e2 e3 e4 e5 e6 name id Employee Entity set MIT 22033, Database Management Systems By: S.Sabraz Nawaz 12

13 E-R Model (Keys...) A candidate key is a combination of attributes that can be uniquely used to identify a database record If a table has one or more candidate keys, one of these candidate keys is selected as the table primary key and the rest are called alternate keys Primary key is underlined EMPLOYEES id MIT 22033, Database Management Systems By: S.Sabraz Nawaz 13

14 E-R Model (Keys...) A key formed by combining at least two or more columns is called composite key Always, the minimal set of attributes are considered for the key. Therefore, A key is a minimal set of attributes whose values uniquely identify an entity in the set. MIT 22033, Database Management Systems By: S.Sabraz Nawaz 14

15 E-R Model (Relationship) A relationship is an association among two or more entities o SaNa works for Department of MIT o Thilan works for Department of MIT A collection of similar relationships is called a relationship set MIT 22033, Database Management Systems By: S.Sabraz Nawaz 15

16 E-R Model (Relationship ) Graphically, EMPLOYEES works in DEPARTMENTS MIT 22033, Database Management Systems By: S.Sabraz Nawaz 16

17 E-R Model (Relationship ) A relationship can also have descriptive attributes These are used to record information about the relationship o E.g. SaNa works for MIS Department frm November In the figure, this is captured by the since in Works_In nic dob MIT 22033, Database Management Systems By: S.Sabraz Nawaz 17

18 E-R Model (Relationship ) 1/2/ /2/99 1/5/01 1/2/87 1/3/95 D1 D2 D3 Employees Departments Work_in MIT 22033, Database Management Systems By: S.Sabraz Nawaz 18

19 E-R Model (Relationship ) An instance of a relationship set is the set of relationships 1/1/ /3/93 2/2/92 3/1/92 3/1/92 D1 D2 D3 Employees MIT 22033, Database Management Systems By: S.Sabraz Nawaz Work_in Departments 19

20 E-R Model (contd.) Degree of a relationship is the number of participating entities in the relationship. A relationship of degree two is called a binary relationship (e.g. Works In) A relationship with degree three is called ternary. MIT 22033, Database Management Systems By: S.Sabraz Nawaz 20

21 E-R Model (contd.) Ternary relationship D1 D2 D3 Departments Employees MIT 22033, Database Management Systems By: S.Sabraz Nawaz Work_in L1 L2 L3 Locations 21

22 E-R Model (contd.) Ternary relationship EMPLOYEES works in DEPARTMENTS LOCATIONS MIT 22033, Database Management Systems By: S.Sabraz Nawaz 22

23 E-R Model (contd.) The cardinality ratio for a binary relationship specifies the number of relationship instances that an entity can participate in. There are three types of cardinality ratios for binary relationships. o One-to-One o One-to-Many (Many-to-One) o Many-to-Many MIT 22033, Database Management Systems By: S.Sabraz Nawaz 23

24 E-R Model (contd.) One-to-One relationship Example: An employee manages at most one department. A department can have only one manager managing it. EMPLOYEES manages DEPARTMENTS MIT 22033, Database Management Systems By: S.Sabraz Nawaz 24

25 E-R Model (contd.) One-to-Many relationship For example, an employee works in at most one department. Graphically, EMPLOYEES works in DEPARTMENTS MIT 22033, Database Management Systems By: S.Sabraz Nawaz 25

26 E-R Model (contd.) Many-to-Many relationship Example: An employee can work on several projects. A project can have many employees working on it. EMPLOYEES works on PROJECTS MIT 22033, Database Management Systems By: S.Sabraz Nawaz 26

27 E-R Model (contd.) Participating constraint specifies whether the existence of an entity depends on its being related to another entity via the relationship type For example, if we specify that an employee must always work for a department. Then we say that the relationship works in is in total participation from Employee entity to Department entity EMPLOYEES works in DEPARTMENTS If the relationship is not in total participation, then it is known as in partial. o For example, from DEPARTMENTS to EMPLOYEES MIT 22033, Database Management Systems By: S.Sabraz Nawaz 27

28 E-R Model (contd.) Entity types without any key attributes is called weak entity types. A weak entity is uniquely identified by considering some of its attributes with the primary key of another entity called the identifying owner. The attributes in the weak entity participating in the key are called partial keys The owner entity and the weak entity participates in an identifying relationship. The cardinality of the identifying relationship is either one-to-one or one-to-many from owner entity to weak entity The weak entity must have total participation in the identifying relationship MIT 22033, Database Management Systems By: S.Sabraz Nawaz 28

29 E-R Model (contd.) Graphically (Weak Entity) EMPLOYEE 1 M Policy DEPENDENTS id name age MIT 22033, Database Management Systems By: S.Sabraz Nawaz 29

30 E-R Model (contd.) Entities participating in a relationship need not be distinct. Such relationships are called recursive relationships. Each entity in the relationship play a role in the relationship. It is recommended to state the role in recursive relationships. manages supervisor subordinate EMPLOYEES MIT 22033, Database Management Systems By: S.Sabraz Nawaz 30

31 Reference: Ramakrishnan, R., & Gehrke, J. (2003). Database management systems. Osborne/McGraw-Hill. (p ) MIT 22033, Database Management Systems By: S.Sabraz Nawaz 31